I'm a big fan of index cards as well. Each potential customer has an index card. If I find out that I should follow up on a certain date (they say "later" for instance), I use my tickler. I also divide my customers according to ones I am working with at the moment (I have a set number that I keep in this category so I don't get overwhelmed and yet have enough new leads that I keep the ball rolling, so to say) and ones I intend to work with soon. I sell a consumable product so regular contact is important but I don't need to be contacting my entire customer base in one week. On each index card, I list the customer name, phone number, and general reason for contact. Then I track my contact attempts and the results.
Other things I find helpful are separating contexts. There are things I do in my office that are direct preparation for doing business and there are things I do in my office that help my business run better but aren't urgent. I have three general categories - planning, preparation and action. Planning includes setting goals and determining the overall direction of my business as well as deciding which customers to contact and the general purpose of the contact. Preparation involves keeping up with things that are part of running by business but do not involve customer contact such as preparing for appointments and maintaining my inventory. Action includes customer contacts - both phone calls and holding appointments. All three general areas need attention, but on different levels and at different times.
Knowing my next action for each customer - whether it involves preparation or action - is key. I find that I don't get much done if I don't know or have a specific plan for what to do next. So next actions are revolutionary in my experience.
That's in a nutshell how I apply GTD to sales. Good luck finding what works best for you!