Prepare, but don't expect
The really good thing about Pessimists is that they don't ignore the possibility of failure. That gives them a chance to explore the possibilities and have alternate plans ready. It's just really important not to EXPECT the worse to happen, because you may actually do subtle actions to ruin your chances of success.

Expect nothing, gain everything.

Enjoy the good times while they last
You believe that the good times won't last, right? But what are you doing while they're here? There's no law that says you can't enjoy the wonderful moments that life has to offer. So stop worrying that the worse is yet to come, and get to enjoying what's here now.

Don't make things worse
You create your own reality by every choice you make. If you believe that something is going to be bad, and you stop trying to make it better, it might actually get worse. You'll feel vindicated in that you expected it, but being in a worse situation won't make you feel any better, now will it? Be aware that you might be making the bad times happen.

You don't need to suffer
Yes, there is suffering in the world, and you should do something about it; just don't feel bad and believe you have to suffer yourself. Starving yourself is not going to feed a hungry child in a Third World Country (unless you mail them your sandwich). Feeling badly about a homeless person is not going to get them off the street any time soon. Don't feel guilty for having more than someone else, or just for having a good day.

By taking care of yourself, the world has one less person who's in pain.