My best tip is to be prepared for cold weather, because sometimes it happens. I mean gloves, hats, coats, the whole deal. I don't mean "cold for Florida" - I mean just plain cold.
We've been numerous times in December and January. Some trips we've worn shorts and swam every day. Some trips we ended up bundled up every day. Often, we did both on different days of the same trip. Our pictures from
MVMCP one year feature us in shorts, sandals, and tank tops. The pictures from next year's party feature us in coats, ski caps, gloves, scarves, turtlenecks, etc.
One January E night several years ago it was so cold that ice was forming around the boarding zone at Splash Mountain and the CMS were sprinkling salt around to help melt it - and yes, a few brave souls were still riding.
Last December we attended the Candlelight Processional on an evening when it was in the 40s. We took a blanket to sunggle under and were the envy of many. That was a really cold trip, over all. We ended up going to Wal-Mart and buying those disposable hand warmer things and putting them in DS stroller so he could sit on them and stay toasty!
My main message is be prepared for anything! It's no fun to walk around freezing to death - or to have to shell out big bucks you weren't planning on to buy warm stuff!
Dress in layers so you can add or remove things as the temp changes throughout the morning, day, and evening.
And, don't think you can skimp on the sunscreen just because it's winter! The sun is still brutal!