To maximize xp gain:

• Buy XP boosts. You can stack duration and per win boosts.
• Play PvP instead of AI
• If you insist on AI, play intermediate instead of beginner
• Surrender if you can't win (to maximize games played)
• Invite friends. Referrals give bonus IP, which help you win (and get more exp)

Exp calculation:

Current (July 2011) formula for XP winning a Normal game at level 15-19 is roughly 3m+25, where m is the number of minutes the game lasts, capped at 198. Intermediate Bot games at level 10-14 use the same formula. At level 15-19, Intermediate Bot games subtract a 25% penalty from that formula, and give XP for only the first 180 minutes a day of play. Blockquote

To maximize XP gain:

• Buy XP boosts. You can stack duration and per win boosts.
• Play PvP instead of AI
• If you insist on AI, play intermediate instead of beginner
• Surrender if you can't win (to maximize games played)
• Invite friends. Referrals give bonus IP, which help you win (and get more exp)

However, I don't recommend trying to level up quickly. IP is much more important for buying Tier 3 runes, and your time is better spent maximizing IP gain.

Exp calculation:

Current (July 2011) formula for XP winning a Normal game at level 15-19 is roughly 3m+25, where m is the number of minutes the game lasts, capped at 198. Intermediate Bot games at level 10-14 use the same formula. At level 15-19, Intermediate Bot games subtract a 25% penalty from that formula, and give XP for only the first 180 minutes a day of play.