While Jillian or Bob or your plastic surgeon have the answers for preventing your actual knees from getting baggy, this little tip is for your pants. 

It's winter, most of us are wearing pants more often than we were a month or two ago.

Lots of sitting causes the fabric around the knees to stretch.  Doesn't matter if it's wide-leg suit pants or super lycra enhanced jeans.  Sit in them long enough and often enough and that area will stretch.

Once that happens, they won't hang nicely while you're standing.  Like this.

You'll lose your crease, if there is one, and it'll be oddly voluminous around your knees.

Luckily, there's a simple fix.  Right before you sit down, hike your pants up just a bit. 
Not so much that you have handfuls of fabric sitting in your lap.  Just let your arms hang down, grab the fabric at your thigh, and turn your fingers under. 
This will be enough to keep your pants from pulling at your knees while you're sitting.

As for this... I know of no such easy fix. 
A million lunges, perhaps?