Anyway, predictably enough, I find experience to be a good teacher. One skill I found especially hard to acquire, though, was readiness to throw out failures. After having spent anywhere from a week to four months nursing these liqueurs to completion, I found that I'd hold on to them for months more (and, more recently, even years) in the desperate hope that they'd get better with age. They almost never did. I've gotten better about it now, but I recommend that you throw out a few early batches that just weren't up to snuff, if only to get used to it. Then, when you make the occasional batch of watery pine sap, you'll find it easier to just toss it. Either that, or start making friends at a local wine supply store so you can get your bottles cheap, since you'll be using a lot of them for storage.
Oh, much more detail on just what steeping in alcohol does can be found at the Science section.
Of the various vodka brands, I have found that the really cheap varieties have a distinctly bitter and unpleasant taste, which becomes part of the liqueurs. I have likewise found that the really expensive vodkas are all equally tasteless when put up against heavy hitters like cranberries or ginger root. Smirnoff® has proven to be a consistently good trade-off point between price and quality. Your other good choice is to take grain alcohol (190-proof or so pure alcohol) and dilute it down to 80-proof. While this is probably the best choice from an economic and taste perspective, I'm just too lazy to do it myself. So I pay a premium to have Smirnoff® dilute my alcohol for me. Some recipes call for 100-proof vodka. I always cheat and use 80-proof, but I expect that Smirnoff® 100-proof is appropriate as well.
There are more than a few recipes, though, that call for something more interesting as the alcohol base. There is an extensive family of flavored-brandy liqueurs, in particular, which use straight brandy as the base, and call for a wide variety of flavorings. But you can make a liqueur out of most any alcohol base, including rum, cognac, tequila, or anything else that might strike your fancy. I understand that steeping flavorings in wine is a popular craft in France, as well. If you feel the urge, you might want to take any of the vodka-based recipes you find, and just try substituting in an alcohol of your own choice. But, alas, I can't recommend any particular brand or variety of other alcohols. I just don't have the experience.
Definitely be thoughtful about substituting alcohols, though. It will change the mixture, and at the least may require a different sweetening. Rum, being fermented sugar, requires much less additional sugar than vodka. Cognac, at the other extreme, needs a lot of help. Every alcohol with a distinct taste of its own is going to alter the balance of the steeping flavors, and you may need to compensate for that as well, by making small adjustments to the recipes. And there's always the possibility of mixing alcohol bases. I have one recipe from a friend for cinnamon liqueur, which calls for two separate batches to be made, one with rum, the other with cognac, and then has them mixed together. Don't be afraid to experiment yourself; there is no "right" taste to a liqueur. You're only aim is to make something you, and maybe your friends, like the taste of. Just don't be afraid to admit defeat if it happens.
However, mixing flavorings usually results in much richer tastes. With fruits, adding a little lemon/lime/orange peel can really liven up an taste. Cloves, cinnamon and nutmeg are also a great addition in moderation. Over-used, they can become fairly exotic tastes, but I've found that I'm the only one who really likes them. Alas. I don't have any general rules for what mixes and what doesn't, since I can't really cook. My gut reaction is that coffee beans should never be mixed with lime peel, but I could be wrong. It might be wonderful. My only hard and fast rule is don't serve garlic liqueur to anyone without telling them first.
One good concept to keep in mind while working on liqueurs, especially if you're trying to make a completely new recipe, is that you're essentially making a dessert. Not that your end product will be relegated to the role of digestif, but the flavoring mix, the underlying principle of the taste, is that of a dessert. Thus, cinnamon and nutmeg go with apples; lemon peel (in lieu of lemon juice) goes with most any berry you can find; coffee needs vanilla. I suppose even peaches need cream, but I've yet to try that particular cream liqueur (though Vargas and Gulling has some intriguing recipes), so I'm leery of trying so strange a combination yet. But peaches work just fine on their own, too. But in essence, you're just coating your flavorings with sugar, so you should make sure that it's a flavor combination that would work even without alcohol.
If desserts aren't your bag, you can also look at liqueurs from the perspective of jellies. Again, the flavor-mixing principles are the same, and even much of the processing is the same. If you have books on jellies, you can learn a lot about what makes a good fruit liqueur. And you might be tempted into some more exotic experiments.
The best rules of thumb I can come up with are:
It's almost impossible to judge beforehand how much syrup to add to a flavoring, since a lot of the chemicals we taste in the raw fruit break down in the steeping process. And you may have some illusions to get over about how tasty your flavorings are in the first place. Many people are surprised to learn that, though raspberries have a strong smell, they have comparatively little taste beyond it. Try holding your nose when you eat them. If you're working with a flavoring for the first time, make a number of small batches with different levels of sweetening first, and then make larger batches of the one that works. And some recipes add the extra step of "sweeten to taste", which it's hard to guess about if you've never made it before.
Another approach to determining the correct sweetening is to try mixing your ratios by hand at the sweetening stage. I found it useful to put two teaspoons of an unsweetened liqueur at a time into a shot glass, and then to mix some quantity of syrup with it to see how it tastes. Once you know just what proportion works, then measure your liqueur, and add just that proportion to the jar, and age. You might, though, also consider putting in not quite as much as you measured out. If your aging period is more than trivial, your sweetened liqueur might well change its composition during that phase. What tasted perfect in August might be completely wrong in October. Since it's a lot easier to add more sugar than to remove what's already there, estimating low in the early stages gives you more flexibility later on. If it turns out that you need more sweetener after aging, just toss it in and age again. You might not even need to age the full period again. Another week may be more than enough time for the additional sugar to settle down.
Many recipes specify a measurement of "simple syrup" instead of the exact quantities of sugar and water. The general recipe for simple syrup is, "2 units of sugar bioled in 1 unit of water yields 2 units of simple syrup." In reality, you usually get more like 2.2 units of simple syrup. Remember that when considering sweetening. The recipe might really mean, for instance, 1 1/2 cups of syrup, expecting you to throw out the extra 1/8 cup of syrup left over. As always, experiment.