What to do with excess fuel before flight?
You have a few options, 

You can burn it off. Laps in the pattern are great for this. Get to the airport before you need to depart and practice some landings. This may cost you in terms of dollars but at least you are not just dumping it.

You can dump it into gas cans if you have them. Every small GA plane i have ever encountered (mostly pipers) has fuel sumps for testing your fuel during your preflight. Most of these sumps can be latched open and you can drain it into a gas can for use later. you should always test for water before doing this. If you rent you should ask the FBO what their policies are on dumping/reusing fuel. I cant find any specific legislation on it but it seems that its common practice (for the airlines at least) to only put the fuel back in the same plane (or within the same fleet) to prevent cross contamination of things that can build up in the tanks. 

The sump on the pipers I fly looks like this 


(source)

Dont have the plane filled up. If you rent planes like many GA pilots do and you plan to take up 4 friends for an evening sight seeing flight ask the FBO to not fill the tanks if someone is using it before you. 

Dont use all the tanks. This will vary plane to plane but some planes have 2 or more fuel tanks which may be independently selectable. You may be able to (weight and balance permitting) keep some of the tanks dry if you know you regularly short hops with big loads. 

For the sake of a complete answer I will mention that planes can be flown over gross weight, I don't advocate this nor would I ever attempt it but it does happen here and there, here is a discussion on it if you are curious.