By "bitter" taste, do you mean more "sour"? If you're getting a sour fluid in your mouth occasionally, ya need to run a soft pipe cleaner down into the shank while you're smoking to sop it up. But moreover, as has been suggested above, dry your tobacco out (to varying degrees) before smoking.
I started smoking a pipe last - jesus - was it october? Man, fast year...
Anyway, I really tried to give it a good run in the months before it became unbearably hot here. But I was smoking a lot of tinned english (the bulk english at my local is shit) and I was always having problems with wet dottle/doddle/doodle (sp?) and a foul-tasting sour liquid that would creep up the stem while smoking. So, after talking with some folks about it, I literally took a tin or two and left them open on my desk for, I think it was not quite two days, mixed them up and gave them another try. The drier backy gave me a much better smoke overall. I wasn't having to pull as often to keep it lit resulting in a cooler smoke with a more even burn, and significantly less (but not "no") gurgle.
So... after playing with it for a while, I'm doing the following:
1. Give everything an hour or two out of the tin/bag and on a paper towel before I smoke it.
2. If it's cased, let it sit all day. It's still likely to be gummy as all hell when you pack it.
3. If I'm planning on smoking from a deeper bowl, that gets extra time to dry out as well.
4. Smoke inside when I can. It's less humid here than a lot of places, but I can still tell a big difference with smoking inside v. outside when I've got my pipe. Not sure why it doesn't seem to be an issue with cigars, but I really feel that pipes are a lot more susceptible to weather, and you'll do better inside than out.
Hope any of that helps. Good luck.