I usually go with vodka and diet coke, but recently I've had a taste for malibu rum. I experimented and really liked malibu rum with a wee bit of squash and sparkling or still water (some of my friends really hate it though, but try it and see if it works for you!) since fruit juice is just as high in sugars/calories as a mixer with malibu.
Pimms with diet lemonade is ok too but I can't imagine having that on a night out regularly lol.
I'm generally ok having pre-drinks/drinks at home cause I can control what liquor I have and what mixers I have them with, but it's quite difficult to find diet mixers in clubs/bars so I tend to stick to lagers... a full pint of Carling is 187 kcals and provided I've already had some drink in my system it usually lasts longer than a short drink of say, whisky-coke.
Anyway those are the usual strategies I've devised and so far it's been good for me (especially considering I live in the northwest of England!)... Have a good birthday night out!
xx
PS: I just wanted to add... It might seem a bit of a drudge to have to limit your alcohol intake to certain kinds and with certain mixers; but it helps for me to remember that it's a sustainable lifestyle change that I want, not a temporary quick-fix diet that will get pounds off but won't keep it off - I know I won't want to cut alcohol out of my life completely forever, so a few restrictions like that is nothing in the long run (and I've got used to it anyway). Good luck!