(Also see Why get an internship?.)
You are evaluating the company (what is the environment like? will you enjoy it once you get there full-time?) as well as the company evaluating you for future employment. You can't lose, for it's only three months; even if you don't like it, you've avoided making that mistake in a full-time job. Internships often turn into job offers (which is what has happened if they ask, "Do you want to come back?"). Because part of the point is to make contacts, you should try to meet 2-3 new people every week.
Know what you expect to get out of your summer, so you can make sure that you do accomplish that.
You will lose a little ground in your research by going away: it will take a little while to catch back up. On the other hand, what you do while away might be publishable, might give you ideas, might make you more financially secure, or might just give you a rest and renewed energy.
Determine upfront what your project is; have a plan for your relationship to the company for the summer. Can you accomplish it in three months? Make sure your project is well-defined. Don't waste time flailing at the beginning of the summer, and watch out that you aren't given some trivial project. Prove yourself quickly and you'll have a better chance to get some interesting work.
Who will your mentor be? Does that person understand your technical work? Your mentor might not have experience in advising over three-month periods. If you're not happy with your mentor, seek out another one, either informally or formally. You'll probably find others happy to help you out.
Different companies have different flavors; getting a taste of many of them can be helpful. Working for a startup is a low-risk way to get that experience. You get to know everyone and know everything that's happening in the company (even in marketing, for example). A startup is very goal-oriented (not as much freedom to do whatever you want), and there is a lot of excitement.
Moving out of town is a big pain. You may end up paying two rents (but you'll still make more money than you would have working at UW as a full-time RA and paying one rent). There are some companies that will help you find a place to stay. Consider storing some of your stuff in your UW office.
It can be nice to stay in Seattle for the summer, but off-summer logistics can be a bit rougher, and you won't have other interns to socialize with. (They are your peer group; the full-time employees probably aren't.)
A longer internship gives you a better chance of interesting work (and amortizing your start-up costs).
Be a little bit careful (and very up-front) about NDAs and conflict of interest between your research and a summer job, or between two different jobs. For instance: do you know what ideas and tools from the summer you are allowed to use in your UW research, and vice versa?
Some companies have good track records and well-developed internship programs. At others that are just starting their internship programs, there will be more administrative rough spots. Stand up for your rights and it will work out OK.
Far and away the best way to get a job is through contacts that you know: through relatives, through a UW grad who works there (and may come back to recruit), through your advisor, through a fellowship the company may have funded. Don't feel bad about this: there are so many applications that even good ones get buried without a boost from someone that the company already knows and trusts who can vouch for you.
Big labs tend to want applications by January; little companies might not want to hear from you until a few weeks before you would start. This is a pain when you are trading off various options. Being on a quarter system is an additional problem. Consider starting early, blowing off the last month of school, to make your schedule mesh better with the company and other interns, or to bring the application/notification dates for small and large companies more into line.
Get your professor to help you: suggest places to apply, recommend you to friends from industry, advise you as to the best summer(s) to get an outside job.
Back to Advice compiled by Michael Ernst.Michael Ernst