I know that your top lip shouldn`t be showing when playing. However It is very hard to play high while covering the top lip completely, is it ok to bare the top lip a little, you know mover the mouthpiece down and tilt the horn forward(down a little)?

Try to keep the embouchure the same, and not go changing positions on the lip, definitely not to a point of having to remove the mouthpiece to reposition...this will never work.

Play where it feels most comfortable for you, there is no correct position. it is the position that sounds and feels good. Concentrate on the middle register, and get a good solid sound there. This will be the best position. Then just practice and use relaxed free air, with pressure to go up. Of course you have to tighten the chops a bit and put enough pressure from the mouthpiece to keep a seal, but don` go squeezing it into the chops too much.

Enjoy it!

This tilting effect is called playing downstream. Many trumpet players play downstream or upstream (playing with the trumpet pointing up a little)and can play great. But my advice is to keep the trumpet as level as possible. Stand infront of mirror and play your horn, trying to get as level with the floor as possible....pointing straight out. Are you finding that you can`t hit the high notes, so you place the horn in this position and then you can? If so its because 1. your lips are not tight enough at the sides so you support the embrouchure by placing the horn at a different angle or 2. you`re not using enough air to play the notes. There is a very good method of breathing that you may want to look into...Bobby Shew teaches it, he learnt from Maynard Ferguson. Its a breathing technique adapted from a yoga technique. It allows you to get the maximum amount of air into your lungs before plaing a note so that it is a full rounded note. Also as you have more air, you can push more air out in one go and therefore play higher! If you want info on this technique email me at sjf04@stvincent.ac.uk.
Hope this helps! just keep practising all the time! Keep it up!!