It's very important at a certain point in a business's growth that you make sure IT is involved in the software selection process. Here are some ways to do that without ticking everyone off.
I've tried a few ways to do this. The first thing is you set a new policy that all software selection must go through a new process. This can have a few side affects. Once the word "process" is let out of the bag, any new software is moved down the priority list. Many managers in SMBs are used to making the decisions and then moving on. There are steps involved in software selection and the time they need to invest in that "process" is more than they have to dedicate. Another side affect has to do with the managers that subscribe to the credo "It is better to do it and then ask for forgiveness than it is to ask for permission." These folks will pull the trigger anyway, so you need to be prepared as to how to address this situation. If it is allowed once without consequence, it will happen again.
Being the heavy on this can also backfire. Putting the kabosh on a purchase because it has not gone through the process can ruffle some feathers. In the beginning, this is typically perceived as a control thing, so if you have to do this, please do so tactfully.
One of the best ways to approach this is to make it part of the capital budgeting process. That way, IT and finance are aligned. A united front is much better then fighting the good fight by yourself. The bottom line is that you need to demonstrate that you add value to the process.
Try to insert yourself in the process in a non-threatening way. Explain all the pitfalls of not going through a process and share real-life stories of how bad things can turn out if these things are not thought through. You also need to explain what you need to do to ensure that the business manager won't have any issues later on. Sometimes they think that forwarding a vendor-provided whitepaper on their solution is enough for you to make a decision. As you try to understand their business in order to provide them value, you also need to teach the business manager what you can do to make him or her more successful. Don't let yourself be designated as just the server and PC person. Get involved with management and sell yourself.