In reply to Simos: I agree with PPP, my DOMS depends massively on what I manage to eat shortly after a session. If I get a small recovery bar (50g bar, 25g of protein in it) straight after my session and then a big bowl of porridge when I get home, waking up the next day can be a lot less painful.
Like you say though, when you get the most aches the next day is when you shock your body into trying something new, as weak muscles you haven't used before all of a sudden have to work very hard! But this is the way to get stronger - continually changing things up so your body has to adapt in a positive way. As mentioned above, happens by to micro-tissue damage (little tears in muscle tissue) which supposedly repairs itself, making it stronger than before - leading to DOMS (delayed onset of muscle soreness - I think...)
Obviously you have to be careful these little tears don't become big tears, so in order for them to repair, don't overdo it when you try new things, give yourself time to recover after particularly bad DOMS, and eat lots of carbs and protein post-session. Not too much - got to stay light too! Lots of things to think about anyway.
So basically, to get strong some post-session soreness is necessary, and the best way to do that is by trying new things, and working your weaknesses.