Your first decision will be whether or not you need a company who can handle not just the printing and shipping of your static clings, but who can design them in house as well. If you all ready have a logo and simply want that added to a static cling, you likely won't need design services. However, you should consider making the clings unique to your company by adding some interesting visual things beyond your logo.
Next, you'll want to find a company that can handle all of your needs for static clings. If you've decided that you need design help, you want to hire a company who has a design team in house. You also want to make sure that you're choosing a company that does the designing, printing and shipping all themselves. Avoid companies who take the contract from you but outsource various parts of the job to other people. You want a single company to work with for the best results.
You'll then want to decide the quantity of static clings that you want. There are two schools of thought on this. On one hand, many people prefer to start with a smaller order to see how well they work or how well their customers respond. On the other hand, ordering in bulk is typically much less expensive and you won't have to deal with ordering for quite a long time. Both options have their pros and cons, you'll need to decide which works best for you.
Once you have your static clings you'll need to decide how to hand them out. Some companies send them to all of the customer's on the mailing list. Others send them out with new orders. Still others simply keep them in house and hand them out when a customer asks for them. If you choose to send them with every order, you'll likely get more of them in your customers hands. However, you can't be sure how many of those will actually be used and how many will go to waste.
Static clings can make great and effective marketing tools for your company. You want to decide if just your logo is enough or if you want extra information added on them. You'll then decide on a company that can handle every aspect of your order, from the design to the printing to the shipping. Then you'll figure out the best way to get them to your customers. Be sure to keep plenty on hand and re-order your clings in plenty of time before they run out.