Here's the best way to...
...start things off Set up a project to keep guests occupied as others arrive and help them warm up to one another. Make a birthday banner, decorate paper crowns, or color paper place mats.
...keep 'em busy Kids whip through activities quickly, so have plenty lined up. In a pinch, use household stuff -- make coffee-filter snowflakes or sock puppets.
...skip birthday-kid meltdowns Nothing's harder for the under-4 set than sharing their favorite toys. Put them out of sight before the party.
...calm the chaos Kids start a party already amped up, so have activities in a couple of rooms if possible so guests don't swarm together in one spot. Follow a high-energy game with a lower-key one.
...get extra help Hire one or two high schoolers. For $10 or $20 each, they can help with crafts and games, pass out cake slices, and spare you from leading the hokeypokey.
...avoid tears Say no to life-size Scooby-Doo or SpongeBob. They're a hit with bigger kids, but little ones may freak out when they see a six-foot character with a giant head.
...make sure the piñata works Prime it with a couple of whacks before kids take their turns, or buy one with strings that you pull to release the bounty.
...prompt politeness If your child opens gifts at the party, have her practice saying "thank you" beforehand.