There is something great about blasting out those cobwebs after a few days of being stuck in side due to rain. Whether the weather plays along or not is a different story. A few things to help you out in wet conditions.
Check the weather
There are a few good ways to do this. Firstly, look out the window, and then be optimistic. In saying this, I am going to contradict my self and say always be prepared for the worst.
Cross reference your chosen weather report. I like to use Metvuw and The Metservice. These are two independent organisations that can give you the same or totally different information. But by checking more than one then you can definitely be hopeful for the better of the two forecasts and still be prepared for the worst!
If all you're getting is rain rain rain... maybe it's best to just go to the garage and treat your bike to a little TLC.
The simplest way to set up your bike for less than ideal conditions
This can make a massive difference to your confidence in wet conditions. It is a bit of a mis conception that your tires must be nice and hard for when you go for a ride. Gone are the days of tubes and skinny tires. If you are running tubeless (you should be these days) then you no longer need to worry about pinching your tube and getting flats. Personally I like to run my tires as low as I can get away with before rolling them off the rim when I rail into a corner. If you find this critical point you are going to get as much traction as possible on the occasional slick corner and those slippery tree roots. For me, I find between 20-25 psi is a good place to start, just add or let out what ever you need. And finally, don't be afraid of adding a little extra sealant next time you put on a new tire. That way you shouldn't have to do anything until you've worn out the side walls and the knobs are nicely rounded.
What to take with you
If it's anywhere in the world that is green, expect that it could rain at any time. Even in the deserts of Utah and Colorado I have been caught out in a rainstorm when only a few hours earlier there wasn't a cloud to be seen. If your taking a bag with you, then you almost almost have a little spare room in it.
Moral of the story, if it's winter, always pack your jacket when you leave home, if you get to the trails and you don't intend to go far and your still extremely optimistic, you can leave it in the car. But if you have space, it's never a bad idea to bring it along. You never know when you might have to stop for longer than five minutes to fix a chain or help someone else with a flat tire. You'll be pretty happy you have this barrier from the winter cold.
In addition to your jacket, a spare thermal top is a must. Even if it's an old one that you use mostly as a rag, they are super handy to have with you, weigh next to nothing and take up hardly any space in your pack.
Know where to go
Save the adventures for when the sun is shining and there is no worry about how cold it is. Take an extra 5 minutes to plan your ride. Remember what trails were super wet the last time you went for a sloppy ride, try to avoid these and you will also avoid doing unnecessary damage to them. Try to think of trails that are well drained, on hill sides, or even in the open. Try to avoid any trails that have just been built as they will still be soft, or trails in little valleys because these collect water. Don't be afraid to ask at a bike shop for advice or to get a guide every now and then ;)