Keep the kids happy and active! Have plenty of games for the kids to participate in and keep the downtime to a minimum. Summer birthdays are much easier to plan outdoor birthday parties for to keep children actively involved, yet it does not mean in the winter a an ice-skating party with hot cocoa cannot be done (in San Diego we are fairly close to snow so maybe some sledding?).
There are also many games that can be played indoors that do not require much preparation and regardless of the time of year for the party, you should always have several backup games for the children to play.
Summer time you could be stuck with a rainy day and winter it could be too cold to go out.
Factor in approximately two games per hour of party minus time for cake and ice cream. Make sure to have enough party game gifts that no matter if a guest wins or loses they will receive a gift. No one likes to lose and after all it is a party and you want everyone to enjoy it and be happy.