Aim to increase the duration that you can work at a given intensity for.

It's not rocket science, just don't allow yourself to get lazy... maybe somebody needs to give you a kick up the arse... we can all look good pot shotting and throwing the odd hard/quick combo on the bag till the cows come home...

If at some point you don't start to look like piss/feel like you're struggling (even if you end up getting a second wind and finishing strong) then you're probably not working hard enough to increase your work rate.

Two words: comfort zone - get out of it.