Let's face it. You've already acquired almost, but not quite enough, gear. Too much in some categories, not enough in some, and not nearly enough in some others. Let's look at these.
Too much -- This category is usually discovered when you try to store one more thing. On occasion, the overflow is kindly pointed out by someone sharing said space. Arriving at this predicament is something to which we should all aspire. Too much gear sprawled about provides unsurpassed camouflage as new gear is infused into the disarray. However, unlike the gear available for purchase, storage space is not infinite. Remember, there's more than one way to be lucky to get half the house.
You can always attempt resale, but there's real pleasure in passing along gear to select aspiring fly fishers after introducing them to the sport. Pass along your knowledge as well as gear. It's particularly rewarding when the prospects' enthusiasm exceeds their financial means. There's also pleasure feeling that when you're no longer around, someone will get out that old rod and fish it for you.
Not enough -- This is simply a matter of re-stocking or building up reserves. A back-up rod or reel, lines, leaders, tippet, shot, standard flies, etc. Organizing is often the challenge here. I like to take the winter to do inventory, fill in, and re-distribute the gear into the proper vest, pack or pouch. If you're a fly tier, get busy. If not, shops often have winter sales. Shops do their best to stock adequate fly patterns, but have to place orders far in advance of the demand. When the season arrives, it may be too late to fill your boxes.
Not Nearly Enough -- The fun part. Assuming you've created adequate space as noted above, you now have time and space, possibly even a little cash, to go for the latest and greatest. Nothing matches the anticipation and excitement of the first outing with a new fly rod. OK, something does, but, if a major problem develops with the rod, you can settle that out of court. Rods and a lot of the new gear starts to arrive at the shops when spring orders come in. This can be a happy coincidence.
The desire for new gear to fill a great void in your closet, if not your life, will be exacerbated by cabin fever. If you haven't been haunting the fly shop(s) go now. Get the gear in hand, try on the new vests and packs, stuff them full of the shop's boxes, put on some insulation, a rain jacket and see if the packs still fit. This may be the time to consider a new rod to take you to the next level or next species. If it is a rod, take the nearest thing you have to the new prospect to compare them side-by-side. Whenever it's possible use the very same line so you're evaluating the rod, not the line.