So about two months ago i started a strong workout routine that has focused primarily on building strength and mass... my cardio routine from the beginning has been lax, i'll admit. and i havent paid much attention to my diet restrictions, but rather increased my diet altogether. I havent gained any fat, which is good news... and i have seen size increases very recently. im 22 and i know from reading and learning that i am still in my peak for body performance, recovery, and growth, etc... here is my recent realization though. i neglected to incorporate an abdominal workout into my routine and have since suffered as a result. i know that large muscle exercises (full body movements vs. concentrated motion) do provide some mid-trunk development. (i.e. flat bench, military press, over-head ext., etc....) but its really becoming a problem that my mid-trunk torso is weak. my posture is becoming poorer i think due to more weight on my shoulders, chest and back from m-mass development. and i also have slight lower back problems i think from stress due to posture.... so my worry is what should i do to correct this effectively? i know that the short answer is to get a good ab workout in... but im kinda afraid to start doing things that i have done before because my abs used to be strong from being a heavily active soccer player (twisting, jumping, running, etc.) now its been over a year since my abs have been taxed significantly at all.... Where should i start??? what types of exercises are good foundation movements that i can do to build my core strength in my torso, specifically my abdominal wall?? I have thought of vaccuum exercises... and i have thought crunches, leg lifts, sit-ups... etc... but what in your guys and gals experience is the best way to start on building the ab wall back up... im confident that i can develop the same results i once took for granted not so long ago... but i just would like to do it right from the beginning... any help offered is greatly appreciated.. thanks in advance!!! 
