Stretching. Seems pretty simple, right? Your muscles are tight, stretch them. Well, proper stretching will help. Stretch improperly, though, and you might end up injured and in worse shape than you started.
The most important rule for stretching is always stretch in a smooth, gentle manner - never sharp, bouncing, or jerky movements. Stretch just to the point of a little pull in the muscles, not to the point of sharp pain. Then hold that stretch for at least 10 seconds (more if you feel like it). Gently release.
Keep breathing while you do this, too. It's surprising how many people hold their breath while stretching. Holding your breath actually causes your body to tighten slightly - no what you want when you are trying to stretch.
After being away for almost a month, I've been spending a lot of time in my garden trying to excavate the vegetables from the weeds. I've been bending over and squatting for hours. My hamstrings have tightened up! And it's begun to affect how I feel when I'm running.
All that bending has also tightened up my hip flexors. A good stretch for that is to sit in a chair. Cross one leg over the other, ankle resting on opposite knee. Keep your back straight and hinge forward from the hips. As you lean forward, you should feel a good stretch in the hip of the bent leg. If your hip flexors are very tight, just sitting straight with your ankle crossed over the other knee might be enough of a stretch.
For these, as well as any stretch, always use proper form. To repeat (because I think it can never be said too many times, given the poor stretching technique I so often see): stretch just to the point of slight pull, hold for at least 10 seconds, and release. And keep breathing!