I worked many years for Kelly Services, off and on. They also have training, and in fact I got my start in office computing through their training program, which gave me several permanent jobs and a couple decades of solid income.
Unfortunately, when I went back to them after my most recent layoff they said they couldn’t get enough assignments for they people they already had, try back later. Well, at least they were honest. I tried signing up with Office Team, but I felt they really strung me along for several months and I got nothing from them whatsoever, not even a phone call the time or two I was promised someone from their office would call me back.
Every local office has a different cast of characters. If one office doesn’t work out try another (if there’s more than one fairly close). If one agency doesn’t work out try another.
Anyhow, I’d say 3/4 of being a successful temp comes down to attitude. Here is what worked for me:
**1) Remember that if you get real money for real work it’s a real job **- treat it like one. Seriously, I saw a lot of people screw up on this one. Professional/appropriate attire (sometimes the job requires you to dress down - when I was assigned to a construction site I needed to wear jeans and workboots, not skirt and pumps), proper grooming, show up on time, be reliable, whenever possible give multiple days notice of unavailability, and so on.
**2) Remember who you work for **- you work for the temp agency, not the client you are assigned to. Do everything you can to make things easy for your supervisor, follow the agency rules, take advantage of training, etc. Last time I worked for Kelly they needed people to be on-call on week day mornings for last minute requests. I volunteered pretty much any day I didn’t have an assignment and there was a slot open. I showed up ready to work. If I was sent on call, bravo. If I wasn’t sent on call I asked for further computer training, thereby demonstrating that I was willing to work AND serious about improving my skills.
**3) Remember what your job is **- you’re a professional temporary employee. That means you need to be flexible. You’re the substitute, the pinch-hitter, the cavalry in a crisis. If you show up expecting data entry and find out what they really need is a receptionist smile sweetly, ask if you can have a minute to call your supervisor (so he/she knows what’s going on - remember, THAT person is your manager, not the client), then say “of course I’ll do it!” Unless, of course, it’s something you’re not qualified to do, in which case definitely call your supervisor. Be willing to do anything legal.
4) Remember that they wouldn’t be calling for help if there wasn’t a problem. You’re going to be walking into everything from a well-considered plan to hire warm bodies to sudden absences due to illness or even death of employees. I’ve worked at construction sites, libraries, in basements, in skyscrapers, in small offices, big offices, foreign-owned companies. I’ve filled in for people on vacations and honeymoons. I’ve filled in for the injured and ill. I once worked at a clinic across the street from a public housing project infamous for shootings and violence where most new hires never returned after their first day (that led to a permanent job). During all this, you will encounter office politics, nice people, mean people, people in a panic, bigots… ALL types and personalities. Smile (grit your teeth if you have to), don’t get any more involved than you have to, avoid controversial topics, and work your butt off.
5) Remember there are a lot of BAD temps - and you will have to battle against that perception and prejudice. Quite often you’ll show up and find someone treating you like a moron or slacker - because they’ve experienced that, or heard about that. Prove them wrong by doing the best job you can with some enthusiasm and good cheer. Remember - no matter how bad it is you don’t have to work there permanently. In fact, you’ll probably be gone in a week or two, if not days. You can handle it.
**6) Remember you have to earn your place in the temp world. ** Your first few assignments will probably be low skill and boring (not always, but it’s typical). As you continue to prove yourself you will be given better assignments. After a year or so I would get to the point where I would be sent out on difficult assignments, interesting assignments, and I would be offered a choice of assignments about half the time. I occasionally got overtime, and earned some paid vacation. Was even Kelly Employee of the Month at one point for basically saving someone’s bacon and making everyone look good, both at the client’s office and the Kelly office.
Temping wasn’t always fun, but I did enjoy it more often than not. Good luck. And hurry up - I’ll be checking back with the temp agencies myself on Monday, so you’ll want to get there ahead of me. 
