If you are a foreigner in the USA with a standard non-immigrant visitor's visa, you are expressly forbidden to partake in paid work and will be deported if you're caught working illegally. Employers are required to establish the bona fides of their employees or face fines, making it much tougher than it once was for a foreigner to get work.
To work legally, foreigners need to apply for a work visa before leaving home. A J-1 visa, for exchange visitors, is issued to young people (age limits vary) for study, student vacation employment, work in summer camps and short-term traineeships with a specific employer. One organization that can help arrange international student exchanges, work placements and J-1 visas is International Exchange Programs (IEP), which operates in Australia (www.iep.com.au) and New Zealand (www.iep.co.nz).
For nonstudent jobs, temporary or permanent, you need to be sponsored by a US employer, which will have to arrange an H-category visa. These are not easy to obtain, since the employer has to prove that no US citizen or permanent resident is available to do the job.
Seasonal work is possible in Alaska's national parks and at tourist attractions and tour companies. Punch-the-clock work in fish processing plants has traditionally been a staple among US students looking for some quick cash. Contact park concessionaire businesses, local chambers of commerce, canneries and adventure outfitters. Lonely Planet's The Gap Year Book has more ideas on how best to combine work and travel.