Some skaters slip a thin, soccer style shin protector inside the front of their lycra running tights to guard the shins against errant blades. Elbow guards aren't necessary. Kneck guards with breast protectors are required for races in Canada. Pants should allow for lots of flexibility at the knee and waist, yet not be so loose that they flap in the wind or snag a skate. Jeans are a bad idea because they're constricting and they chafe.
Sweat pants are good, and lycra running tights are the choice of the fast and the fashion conscious.
Wear long underwear under them. Use layers and dress for sweating in a cool rink. Try not to pick something that is so loose-fitting that it's wind resistance slows you down. Don't forget to pull back the hood at the end of your race for the cameras. Bring a rag or small towel to dry your blades, walking guards to protect them 'till you're on the ice and a water bottle to keep you hydrated.