Direct Experience Matters Most
Not just any lawyer can help you form your business. Even if your venture is very straightforward, make sure the attorney you choose has experience in getting businesses started. If you plan on raising capital, particularly equity capital (from family, angel investors or venture capitalists), it is even more critical that you hire counsel who has dealt with these types of funding arrangements.
Make Use of Referrals
Talk to other small business owners; ask for advice on counsel they've used. Use Google to check for attorneys in your area who have worked with start-ups. Finally, search Avvo for attorneys experienced in business formation. If you plan on raising capital from VCs, you should consider hiring counsel in a VC-rich environment (e.g., Silicon Valley, Boston, Seattle) even if you and your business are located elsewhere. You need an attorney with a lot of direct, recent experience dealing with VCs, and you will have a much harder time finding one outside of a VC mecca.
Take the Time to Talk
Interview your leading contenders - don't be content to go with the biggest name, best resume or hottest experience. Besides having the requisite experience, you need to make sure that the attorney's style is compatible with yours. Odds are, you'll want someone who is business-focused and will be able to appropriately priortize legal issues against your risk tolerance (and, most likely, limited ability to pay). You probably don't want a "lawyer's lawyer" who will run every issue down with equal fervor. So - sit down (or get on the phone) with each lawyer and run through some of these "style" questions. Ask about experience, and ask how they negotiate with providers of capital. Ask how they would form your business in the most cost-conscious manner. Ask about rates, and ask who (partner or associate) will be doing the bulk of the work on your file.
Hiring a Lawyer
You'll sign a letter of engagement with the lawyer you choose, but the more important factor is making sure you've got alignment on approach to legal issues and negotiations, scope of the work to performed, and expected response times. Particularly if you are raising capital, it will be imperative that both you and your lawyer understand the importance of very quick responses to the others' phone calls and e-mails.