Having lots of apps installed will not affect your battery.
Having multiple apps loaded into the RAM will not affect your battery unless they are actually doing something (like playing music while downloading from the internet and playing a game all at the same time)
If an app isn't doing anything then it will not use power but if it remains in the RAM then it will start more quickly when it is needed.
Killing apps will therefore not help your batter, in fact it could have an adverse affect because the apps will use more power to start up next time.
Keeping wifi on constantly will drain the battery much quicker, especially if you use it, but even when the player 'sleeps' it will still be active (as you say, finding hotspots and syncing emails.
Using the brightest screen setting will drain the battery
Using live wallpapers will help drain the battery.
Using active and animated widgets will help drain the battery