If you are new to the idea (or sport as some like to say) of dumpster diving here are a few guidelines to help get your started:
1. Be Safe!! You never know how people will react if they see you loading up your car in the dim light of evening. Always be aware of your surroundings and it helps to have a friend with you. It's usually best to do this sort of work during the day. Also make sure this isn't against the law in your area. Some towns do not allow this sort of thing.
2. Be courteous. Usually if something is set out in the front yard near the street or by (or in) a dumpster it's usually fair game. But if it's leaning up against a wall or fence that's different. When in doubt sometimes it's ok to ask if you're in a neighborhood or area you are familiar with. Sometimes people are willing to part with stuff or they may be hanging on to it for another reason.
3. Be smart. You can't save everything. So really inspect that table before you toss it in your trunk. Can it really be saved? Will that funky smell ever come off?
4. Be Savvy. Know when the heavy trash pick up day is. Check around apartments at the end and beginning of each month. Sometimes people toss what they can't load in their truck. Colleges are also good places to search.
5. Keep gloves and even closed toe shoes in your car in case you see something you have to stop unexpectedly to grab. Gloves come in handy because things are sometimes covered in dirt and even glass. Closed shoes are good to have because you never know what is lurking in that wood pile. Personal experience and why I should follow my own advice: I was bitten by something (a spider?) on my foot after one recent dive. Long pants don't hurt either.
6. Clean it up. Clean your new treasure to make sure nothing nasty is hanging around. Also clean yourself! You never know what might be clinging on your pant leg...