?

Have interesting content and those SEs will O themselves automatically.

Choose a company that

A) has a high ranking themselves
B) can show you case studies of what they have achieved for their clients
C) can offer insurance / can guarantee that any changes they make won't get you blacklisted
D) if necessary, contact a couple of their clients to see if they are happy with the results/service
E) find out exactly which search terms they will achieve ranking for, and make sure it is in the organic free results, and not just Pay Per Click

Hope this helps