Best thing is to just relax and be yourself. Being loud, nervous or looking uncomfortable is probably the thing which will get you bumped more than anything else. A few words of German won't do any harm. I've rarely been asked anything and it's best to just be honest about your language skills !
On clothes I found discreet at the door has worked for me (put what you like on underneath lol), but you don't need to wear all-black and I've never gone in looking "hardcore". Garish "Fruit of the Loom" / "Primark" looks probably won't do much for your chances - just use common sense !
Once you have a stamp you can leave and return without paying again for as long as it doesn't wash off. Of course if you turn up plastered / wasted they might bump you on re-entry but if you're well behaved and in control that's not normal. There is a separate queue for re-entry and in really busy times you may still have to wait a bit. The biggest issue is the time it can take to get through the coat check and get your dog tag. Seems like it takes forever every time. So going back in isn't necessarily a case of breezing up and waltzing through.
I really suggest going late Sunday morning if it's your first time - have something decent to eat before going in if you can for stamina. I've turned up there several times as the only person arriving or behind 2, 3, 4 people only. Walking the full length of the zig-zag crowd control barriers on your own in broad daylight up to that building being watched all the way by the doormen from behind a door which looks like it's closed is an experience in itself
... but plenty of time to get yourself focused and reconciled to whatever your fate is to be ! The place is amazing at that time of day - stepping into another world from the daylight outside.
Good luck and have fun.