We cold-water divers have it lucky! We can always dive.
If you've been out of the water for a while, taking a refresher course, or at least a pool session with your local dive shop is a good idea. You should get your regulator inspected and serviced, too. If you own tanks, get them to a shop for visual inspection (and hydro testing if they are due for it). Put all of your gear together and check for leaks, then test each of the functions on your BCD. You can also have your dive shop inspect and test your BCD if you're not confident about it.
Make sure that your mask strap, fin straps, mouthpieces and so on are in good condition, not cracked and dry.
Check the batteries in your lights & cameras.
Try on your wetsuit to make sure it still fits!
Make your first dive back in the water an easy one, to make sure you're comfortable with all your gear, and do a weight check at the beginning to ensure that you have the proper amount of weight. Even if you're wearing exactly the same gear as last season, your body may have changed during the time you were out of the water.
