Here are the areas you need to be clear on before engaging in a monthly SEO service.

Primary and Secondary Keyword Phrases
You need to choose the keyword phrases well before hand, and then you need to have a conversation with the SEO company about the realistic chances of getting a first page listing for the terms.  Be wary of the firms that will just say yes to whatever you want.  You want to deal with a company that knows the competitive lay of the land for your industry, and is able to tell you when you are reaching too far.

Time Frame
You need to find out what is the realistic time frame to expect to see results.  Set out dates that you will review the results so that you can make sure the relationship is working out.