This is a pretty basic tip, but one that took me a bit of time to figure out when I first started out as a GM: The more copies of the rules you have on hand, the better.
For character creation, one copy per player is ideal.
During the game, having at least two copies around is handy — one for you to reference, and one for your players to use.
Between sessions, everyone needs enough time with the rules to choose new abilities, plan for the next game, level up, etc. Again, one per person is ideal.
It can be cost-prohibitive for every player to have a copy of the core rules, and while it’s ideal it’s certainly not mandatory. You can have just as much fun sharing one dog-eared book as you can with five copies.
PDFs — and printouts you make from those PDFs — can be a great way to save money in this department, though. If you have a laptop on hand during games, both PDF copies and online versions of the rules (like D&D’s SRD) can be readily available — and in the latter case, usually free.