Don't want to sound discouraging here, but you should start working with your domain ASAP. I had also bought a PR3 domain last year. After 2 months its PR dropped to zero. I had actually parked that domain and did nothing with it until I saw the drop in PR.

Moreover, PR3 domains are not that costly guys. I have bought mine in $45 from tdnam auctions. You can even get PR4 and PR5 domains in a reasonable amount, but from my experience, one should only spend his/her money in buying high PR domains when he/she knows that PR will be maintained by him/her.

For promotion, I will suggest you to start putting more content in your site, and start building one way links for it to keep that PR stable. Moreover, you can sell links over your site to other advertisers.