Core strength training

I'm curious about what kind of programs people are doing for their core workouts. I've been going to the gym since August for upper body workouts, alternating triceps/back and biceps/pecs for a total of four days a week. All my workouts incorporate core training, and I definitely can see improvement in my abs, but I'm wondering about whether I'm doing the right set of exercises at the right level of intensity. I'm generally pretty fit (at my ideal weight) and 45 years old. So here's what I do:

--crunches with my feet on a ball: 20 reps x 4 sets
--planks: 45 seconds x 3 sets (with cobras in between sets)
--bent leg lifts in the captain's chair: 15 reps x 4 sests
--back lifts (those things that make one look like a ship's figurehead): 15 reps x 2 sets, with a 5 lb weight
--side sit-ups for my obliques (on the same support as the back lifts): 12 reps x 2 sets per side, with a 5 lb weight

Question on the planks: I hold the position quite flat, but in the mirror I look like I'm hunching my shoulders a bit, and I feel stress in my lower back. Is that normal for the position?

So, what do you think? And what's your core program?

2x/week-
Planks: 2 sets 1 min each, w/ 30 sec of holding 1 foot up.
Side planks: 2 sets of 12 up & down.
Twist on ball: 2 sets 12 each side.
Walkout pushups on ball: 2 sets of 12.
Knees to chest on ball OR pike on ball: 2 set of 10.
Cable punches: 2 sets of 12 each side at 25#.
Bridge on ball: 2 sets of 10.
Bird dogs on ball: 2 sets of 10 each side.

1x/week
Pilates

1x/week
Yoga

2-3x/week
When swimming lots of dolphin kick, usually about 15-20 laps.

I change this up about every 6-8 weeks. No pain w/ the planks for me, do you do them on your hands or elbows? Make sure your abs are kept tight and you are breathing.