I do yoga 1-3 days a week in the a.m. (cardio and strength at night) love it, increased my flexibilty three-fold.
I don't have enough time to devote to stretching properly and fully after cardio or weights, although I do a 30-second stretch per body part worked.
I started yoga about three years ago when I was 31 for exactly the reason you stated--I started feeling unhealthfully tight and stiff in the mornings and after workout, and my flexibility was significantly reduced. I was also starting to get sports injuries for the first time in my life.
I think it's a good idea for any regular exerciser, esp. over 30 yrs old, to devote at least one 30 minute minimum session/week for a good, overall body stretch. If yoga is not your thing, there are a number of all stretch tapes/dvds that do this- Tamilee Webb, Karen Voight, Millenium Stretch (scott something)...they include proper warm up so you can stretch safely and properly....
It doesn't take much, you just have to be consistent.