We have just done a bathroom (well a professional company did it for us) and they're coming back this year to do our second bathroom.
Fantastic tip above about the controls being in the side near the door opening for the shower! It's things like this you don't think of until after, and it's where a professional bathroom designer comes into their own. Wouldn't have even occurred to us.
The best thing in our bathroom is the mirror, it's a simple rectangle with a touch button on the front so it illuminates all the edge, and it's heated behind it when on so stays clear when in the shower. The light is amazing and makes it very good for putting make up on and doing eyebrows etc. so my tip is to get a decent quality mirror.
Think about storage. We made an alcove behind The wall so have a Storage cupboard which is flush with the wall.
And don't be afraid to move things around, don't automatically think you have to keep the same layout. Our designer suggested a new layout which meant things were better positioned, there was actually more floor space but we ended up with a bigger shower too.