Of course, your situation is not as mine was. If your undergraduate professors don't remember you, then it will be hard to get good letters of reference. In which case you need to develop or utilize your current relationships to get good letters of reference. For encouragement though, a brief telling of my experience follows.

I spent 5 years in industry between leaving my undergraduate institution and entering graduate school. I still had professors who remembered me and were willing to write letters of recommendation. That, combined with luck, a system for mass application (to over 70 programs, essentially down to 4 that I liked and 1 I really cared to attend), and a strong personal letter indicating a strong motivation for bringing my work experience into degree studies, was instrumental in my securing entry to graduate school. I did not need to provide references from work, but I was prepared to supply those.

One thing I could have done to improve my chances was to talk to some members of the department I liked and find out what parts of my situation would help my application. Igor Rivin seemed especially helpful in his comments and offer of time; if you get some one like that at the department you want to attend, pump then for all they are worth and repay them as appropriate, usually by thank you letters, acknowledgments in publications, and small tokens of candy or liquor or textbook or other appropriate treat. (Do this in a socially acceptable way, and not like bribing a member of an admissions committee.)

Gerhard "Ask Me About System Design" Paseman, 2011.01.24