Yep, full press ups (I went through Phase 1 end of last year). The PTIs hate seeing people put their knees down, though I reckon they're a tiny bit kinder towards girls than guys. I was ok, but not great at press ups, but as long as you long like you're giving 100% and if you really have to 'rest' stick your bum in the air then carry on, rather than putting your knees down I found I got by without being shouted at. I never really got good at the narrow / tricep type press ups, but again I didn't get shouted at for wide arm press ups because I mostly kept going rather than stopping. The boys seemed to get more hassle to keep their elbows in, but I'm guessing they could do better press ups in the first place (I wouldn't know, I was just looking at the floor trying to ignore the sweat running into my eyes!).
It's definitely worth practicing them to get as good as you can because the first couple of weeks you will be given press ups as a punishment for everything. Our first session doing the fitness test we racked up 3 lots of 25 for people talking and fidgeting.
As well as press ups, the most useful thing you can practice is short sprints up hill. Camp sprints is an 800m sprint up two short hills in under 4 minutes. It starts off as a punishment, but it's all just practice for your final IMF pass out, so unavoidable.