I'm not a pro, but have done both of mine.
What I think is helpful is to chalkline in a grid that is level with the tub top and perpendicular to it. This will help keep you square. While you are tiling measure, measure, and measure again....be sure you are square.
Also, next time I do a tub/shower, I will do the bottom row first all around the tub, let that setup overnight, then finish the rest of it. This, I think will give a nice base to build the rest of the tiles off of. Otherwise they have a tendency to shift.
Lastly, buy or rent a GOOD tile saw. Don't go cheap. Get a good blade and don't force the cuts, let the saw do the work.
Oh yeah, 18 inch travertines are fucking heavy and hard to do yourself!!
Oh yeah two. I found that spreading a very thin layer of thin set on the tile as well as the usual on the wall helps the tile stick better.
These are the things I found doing mine. The pros may have some professional advice though.
Hope that helps.