Amadeus,
I am definitely an ectomorph. I have thin legs and arms, and have always had extra flab on my back and abs. I am almost 5'10", and weigh 130-135 pounds. My arms have been getting a bit more definition, and my stomach looks better than ever now that I started using less cardio during the past winter. My daughter told me that my arms don't look skinny anymore, and it seems like the flab doesn't hang as much over my bra! I am far from having six-pack (at 42 with two kids, I'm OK with that!), but I can see a little bit of definition in the abs. I've actually gained a couple of pounds, but my waist measurement is the same.
I started using the S&H DVD, doing one workout on M-W-F, and a long cardio session on T-Th. I alternated that type of week with a week of endurance weight training (like PH or MIS) on T-Th and long cardio on M-W-F. After doing that for a while, I noticed major improvement. I've been working out for 9 years, and this routine seems to work for my body type.
Now I'm doing a modified tanktop rotation. For example, I'll do 30 minutes of cardio with CTX Shoulders, Biceps and Triceps on Mon., 30 minutes of cardio with PS Legs on Tues., PS Upper body on Wed., long cardio on Thu., and something like MIS on Fri. That's only 120 minutes of cardio each week, and I have continued to see my arms get more defined without gaining fat around the middle.
By the way, I also think that kickboxing and planks have helped with the tone around my core. I notice that when doing sidekicks and roundhouse kicks, my core tightens up with each kick. I think the kicking and the various Cathe plank routines have made a big difference in the firmness of that area. I'm sure diet also plays a role, but that is the hardest for me to follow!
I'll go back to the S&H type rotation in the fall. It has been the best for me.
Sandi M