In church this weekend, looking at the fit on the woman in front of me, I thought of something else: Is the WB actually too big and your pants are sitting too low on your hips? Pull up and pinch in the waist and seat. If you can, take that in.

Answer for "mclones1": If I understand you correctly, the crotch is too high/rise is too short (the distance from the WB to the crotch). Let me guess; does the CB waistband pull down? If flatfelled seams are not involved, it's an easy fix (but tricky to explain). To lower the crotch, you actually have to "take it in" by stitching a deeper "U" and definitely trim the seam allowance. Experiment in small increments so you don't end up with the crotch to your knees.

Okay, okay. Are we talking strictly about a droopy seat, or is the seat just plain too big? Two slightly different problems.
If it's actually both, have a trusted (better yet, skilled) person stand behind you and pin, taking in the center back seam until it's smooth over the hips, then HORIZONTALLY pin each leg where it meets the seat (i.e. the crothline) the amount it takes to make the legs hang straight to the floor. This one is only a measurement. Compare the two; they SHOULD be the same, but if not, split the difference.
Chalk your CB pinline inside, remove pins and stitch, tapering up to the WB. Leaving the CF seam alone, rip the inside leg seams to the knees (typically 12"; often there's a notch).
NOW, measure IN at the BACK crotch from the seamline the amount you had pinned horizontally. Taper a chalkline from there back to the knees, and stitch.
Create a new crotch seam AS HIGH AS POSSIBLE into the seam allowance, matching CF and CB; this may take some adjustments. When you're satisfied, stitch everything again and trim all seam allowances. Now you can walk in public confidently. No one looks good with too much fabric hanging off her backside!

To "Kate13": THANKS for using a tailor! While I understand your desire to avoid costs, this is how I make a living and why I spent four years in college to get a Bachelor of Science degree in Clothing & Textiles. Please do utilize our skills and don't feel you must "reinvent the wheel". It's nice to be needed.

My biggest distress is trying to match thread colors for garments made in years past. I own a lot of thread of dubious vintage/strength, just to be able to alter and repair older clothing. Yes, new palettes are fun, but whew!--a lot to keep up with!

Or you could just do what manufacturers do (check the kick pleat of your winter coat): on the inside, after hemming, start at the hem fold and (backstitch 1st) stitch 1/4" in from the fold of the pleat, angling up to the pleat edge just above the finished hem. It's easy, fast, and keeps the pleat always in place.

I find it easier to do exactly opposite: keep the body of the garment towards me, and catch the hem on the away side. This is also the stitch to use for a pinked (catch 1/4" down) or seam-binding-edged hem. Keep the stitch under 3/8" total.