The proper design of a "metcon" or any conditioning, relates to your weaknesses and your goals.
If you're a firefighter or soldier, then your conditioning might involve buddy carries, or carrying heavy shit up stairs, odd-size device pressing and throwing (think sandbags), and what not.
If you're a football player, then 40 yard sprints and sleds or prowler exercises would be a good fit.
If you're a chunky guy, and just want some conditioning to go with your strength training, a mix of bodyweight exercises, sprints, barbell couplets would satisfy this "need".
If you're a skinny bastard that needs conditioning so you "won't get fat", Forget it. Don't worry about conditioning!! Go to 70's big.com and get inspired.
To work with a strength training program effectively (aka, won't burn you out), the conditioning needs to be short (under 10 minutes) and involve a lot of musculature.
KSC posted a very good way to design these in his log http://startingstrength.com/resource...t=8331&page=70
ideas here also in my log: http://startingstrength.com/resource...8&postcount=39