There really is no getting away from the fact that alcohol has calories. But unless you are getting smashed, it doesn't have to be too many. Figure about 100 calories per 1 1/2 oz shot of booze or a 5oz glass of wine. Stay away from mixers with calories and you should be able to stay within your diet. (Note that unlike wine and booze, there are significant non-alcohol calories in beer. A non-lite beer has about the same amount of alcohol as the 1 1/2 oz shot or the glass of wine, but will ring in at 150 calories.)
I'm not a big fan of "diet" mixers - diet tonic, diet coke, nutrasweetened margarita mixes, etc. You end up with tasty drinks that are too easy just to suck down.
Seltzer works well as a mixer - gin and soda's go down much slower than run and diet cokes. Seltzer is also great for cutting the calories in wine - a wine spritzer with 1/2 wine 1/2 seltzer is very light on calories.
Keep in mind the biggest risk with drinking - that you get a bit tipsy and start chowing on food!