One muscle that often becomes tight is the lower leg. Performing a calf stretch on a regular basis is the best way to both prevent this and treat it. It really only takes a second to perform a calf stretch and is something you can do anytime, whether you are talking on the phone, brushing your teeth in the morning or waiting for dinner to cook. Getting into the habit of doing this whenever you think about it will help to decrease soreness and also prevent shin splints, as they are often the result of tight lower leg muscles.
To perform a calf stretch, simple stand facing a wall about a foot away. Then prop one foot up so it is against the wall and lean inwards. It may help if you bend the leg you are standing on slightly but this is not necessary in order to do the exercise. When you feel a slight pull in the leg region, stop and hold this position for at least 15 seconds. It's important not to jerk your movements at all as that could risk tearing the muscle and causing further pain. Afterwards, switch feet and perform again.
An alternate way to do this is to hang one foot off the edge of step. While holding onto a railing or something else for balance, slowly let your body sink downwards. Since gravity will be acting on you, you will really need to watch out that you do not let it pull you too far down. This exercise is slightly more risky than the former since you aren't in quite as much control with regards to how much stretch you get.
If you really want to get a good workout in before performing the flexibility work, first do a few raises in this stair position. Since you will be increasing your range of movement you will stimulate more muscle fibres. Just be sure on the last rep to pause at the bottom and allow the lengthening process to take place.