I also used to mow lawns for money, usually getting $40 from it plus a $20 allowance. Same deal in the winter, except snow shoveling. An even better way to get cash is if somebody in your family owns a small business or something. My uncle paid me about $50 a day to move boxes and do other random shit on his property when he needed some extra help. If you find out a relative owns a business just let them know you might be able to lend them a hand, get some money for 'books'.

In desperate moments you could always get all the spare cans around your house, maybe ask friends and in the extreme case look on the side of the road. A garbage bag full of cans could yield up to $10.
If you have enough spare change go to a CoinStar or roll the coins and go to a bank. This works best in combination with can collecting.

If you have an N64 and just got a new xbox, or CDs/DVDs you don't listen to/watch anymore then pawn that shit. No shame in pawning old electronics that were rightfully yours but you don't use anymore.

Many purchases for computer accessories and software come with rebates. Assuming the family member who bought the item was ignorant of it or just not motivated enough to claim it, do it yourself. It takes time but sooner or later the check comes in the mail.

Also, ezjim mentioned that some dealers will front you shit to help you start selling. Beware, it isn't hard to smoke everything you're given and end up several hundred dollars in debt.