Ditto what everyone said about testing as many as you can get your hands on.
Go to one of the larger quilt shows and visit every single Long Arm booth. Try all the machines out in every size available ... even the sit-down machines (mid or long arms mounted on tables as opposed to a frame). You may prefer to move the quilt under the stationary needle as opposed to moving the machine around the frame. Almost all of the big long arm companies have sit-down models now.
When you test the machines consider ... how easy do they glide, are the larger ones too heavy for you, test your "reach" (a 26" machine is longer than I want to reach across a frame, I'm only 5' tall), can you get an easy feel for things like stipling, straight lines, and micro work, does the machine have the features you need and want, ditto for the frame - ask how it adjusts, can the machine be upgraded (ie buy it initially without things like stitch length regulator or lasers, then add them later).
Take a notebook with you and don't be afraid to jot notes while the dealer is talking.
I did all of the above recently at the Chicago International show and one thing I noticed was that the machines that I tested early in the day were not as easy to use as the machines I tested later in the day. I suspect that this is because as the day progressed and I got my hands on more and more machines, I became more familiar with operating a long arm (it was my first day using long arms!!). So ... if this is you ... be sure to go back to the first machines to see if you like them better after you've had a chance to become comfortable with them.
Do google searches everywhere and check out all the reviews, complaints, etc.
I have made my choice based on the features that I like, the reputation the company has, the service reputation the company has, and how much I liked the machine. I will hopefully own an Innova by the end of the year.