I feel your pain and as a fashion photographer I have to do a lot of free work at times.
The key to this is simple though, don't do free work (or cheap work) for commercial businesses that stand to profit from your work as this devalues you.
Instead find interesting projects to build up your show reel.
Approach charities and pitch them ideas to shoot.
Collaborate with other creatives and create short films for online editorials.
Come up with a personal project and shoot it for yourself.
All the while make sure anyone you work with knows your "standard fee" is of significance so that when you get word of mouth referrals you get the right kind.
