In order to validly pull you over, the police need "reasonable suspicion" of criminal activity. That "crime" can be anything, even something as minor as a traffic offense they're investigating. If there was no valid reason to pull you over, then any subsequent evidence, including the observations of you behind the wheel and any statement you give regarding your suspended license can be suppressed by the judge.
No proof of service
The DA must prove that you knew your license was suspended at the time you drove (the "knowledge" element of the crime). There are many ways that you can be notified. The court, police or DMV may have given you a written notice at some point that your license is (or will be) suspended. If given directly to you, it's tough to argue you didn't have notice.
If the notice was mailed, there is a presumption that you got it (although we all know the post office messes up time to time), but you may have some wiggle room, depending on when and where it was sent.
Your DMV history may show that service was attempted, but incomplete. Unless you made a statement to the officer that you knew your license was suspended, you may have a decent shot at beating this charge with incomplete service.
Did the suspension end?
Often, license suspension is for a specified period of time. If that suspension period ended, but you didn't go get your license reissued, you may be only guilty of a lesser charge of driving without a valid license.
Can you get your license back?
This is probably the single biggest thing you can do to help yourself in a 14601 (driving on a suspended license) charge. If there is any way humanly possible to get your license reinstated by the DMV before you go to court, do it. This gives you one of the best possible chances to help your attorney get the charges reduced or dismissed.
Do I need a lawyer for a suspended license?
The short answer is yes. Although they can be charged as misdemeanors or infractions (in some cases), they count on your driving history and can be used as priors in the future. As misdemeanors, they carry the potential of jail time, probation and huge fines. You're always better off with legal advice in this situation.