Post-Workout Muscle Soreness

What are some tips for reducing sore or tight muscles after working out?

I have just begin my training (to lose weight AND Tri), and yesterday was my "rest" day.  But I had to grocery shop...I ended up being there an hour or so (as usual - I don't rush or I forget things, plus I sometimes like to peruse the shelves), but for some reason this time my legs and just overall body was sort of tight, like my neck felt tight and my leg muscles were tight.  I stretched some at night , but I notice that this is a common issue with me  - not just with minimal walking like at the grocery store...but after a workout (of walking on a treadmill or playing some tennis).  Even muslces I didn't think I used were tight (my back and neck were tight when I just walked).

So are there specific stretches I should do as soon as I am cooled down (or come back from a workout)?  Should I be doing more stretching as the night progresses?  A lot of times I am trying to read or study and I get a headache from the tension, and I find stretching doesn't help, so I take something for the pain.

Any ideas on this would help, including a good book or two that could be a good intro into muscle pain or health.

Thanks