When I was Middle/High school, I used to cut lawns (Classic, something like $25 per acre works nice, most people will know their yard size and giving a rate per acre will make you sound more sophisticated for your age and more likely to take you seriously. Remember buying gas is a priority when cutting lawns.), lay brick patios, fix the local older people in the area's computers (You'd be surprised, many people in the 50s and older are willing to give $50 just for simple fixes, upgrades and repairs, clean people's yards (I never clean other's houses as I've never liked being in random people's houses out of nowhere. Many older families whom's kids have moved away can no longer do the work themselves and appreciate the help. Don't ask for money from them, do a good job, after a few times I can almost guarantee they eventually will offer you money.) Wash cars (This is a good one.)
Heck, I even filmed football practices and games for the local high school when I was old enough to work. I made nearly 2 grand working 6 hours a day in the early summer, 3 hours a day 3 days a week during the football season and 4 hours for each game for 2 months. (Plus I did extra like film like Freshmen games for the Freshmen coaches and JV games which normally are never filmed got me bonuses on top of the hours.
Trick is to do what you can and know how to do and do it for a reasonable price. There is always someone near by (Well as long as its semi populated where you are) that remembers being in the same sort of situation growing up and is willing to return the favor to the next generation, and remember where your older and if you have some money you can spend, to try to return the favor to the next generation as well.