So I have been working a lot on my portfolio the past few months, reformatting, back sketching, etc.
I think a great way to improve any part of your portfolio is to get feedback. There are a ton of places online that will help you out you just need to put up your work. Additionally don't be a dick when people critique your work, if you don't agree with the advice then don't take it but getting into some stupid e-fight, well no one wins.
A short list:
core77 portfolio forum
product design forums portfolio forum
id sketching forums (home to 4 designers that are better than you)
I strongly suggest posting work on any of the above forums, its a good way to introduce your work to the id community at large without a job hanging in the balance.