One tip. You are not picking an agency, you are picking an agent. Every agent is an independent contractor basically running their own business (within limits and guidelines, of course). Every agency has good and bad agents on the roster.
How do you pick one? Simple.
1-Call an agency that obviously does a lot of business in your area.
2-Ask to speak to the office manager.
3-Request the name of a FULL-TIME agent with at least 5 years of experience.
4-Interview said agent to determine chemistry and marketing program.
5-Get 5-10 recent references.
6-Call 3 references and verify competence.
7-If all is well, select agent and sell home.
If any step fails, rinse and repeat with another agent.
Another tip: You want to find out how the agent is going to market your home right? But they all seem to tell you they do everything. Well, when you interview the agent, ask for listing sheets for one or two of their currently listed homes. Now go online and see if you can find that home from the standpoint of a potential buyer. Check out the online presentation and evaluate the quality of the photography, video, description, accuracy, etc. It the agent's listings don't look better than all the rest, then proceed to selecting a different agent.
Selecting your agent is important, so it pays to take extra time and effort to make sure you are getting a good one.