It depends on the extent to which you'd like to be involved. But, most large financial companies will handle the rollover for you.
The financial company that you choose will give you some forms for your companie's 401K administrator (HR for most) to fill out. It instructs your company where to direct the funds.
If you're reasonably familiar with investing you could do a "self-directed" IRA (Vanguard is a very good example). You can invest in index funds and market segment funds (heathcare, technology, natural resources etc.).
The benefit of Vanguard is that, since you do your own picking of funds, you can avoid paying "loads" on your purchase of those funds. All Vanguard funds are "no load", which means you don't pay a sales commission for that fund. Loads can easily run 3-5% up front. So, if you roll over 100k, you can end up sacrificing $5k right up front.
Of course, if you feel more comfortable using an investment "advisor", just remember that you'll pay for it. However, you may be able to negotiate with your advisor right up front, with respect to load fees. Mutual funds offer these "load" incentives to brokers/advisors to promote their product. BUT, there's nothing that says that the broker/advisor MUST take the full amount of what the fund pays him (i.e. 5%). So, you may be able to negotiate that you only pay 2%, in turn for some direction from the advisor.
You MUST bring up the load question/advisory fee issue right up front whenever you work with a financial advisor. Personally, I think that you could easily self-direct your rollover and pay a very minimal (if not zero) amount of fees. Sure, there will be a set rate ($200-300 maybe) for transaction and paperwork handling. But, the big expense comes when you actually invest in a mutual fund that may be loaded (note: there are different kinds of loads-front, back etc.).