Switch to Red Bull! :) Seriously though, a few years ago, I did a similar exercise, and found that Yerba Mate worked REALLY well for me. It's got caffeine, but without the nasty side effects of regular caffeinated beverages (the crash, the withdrawal symptoms--at least for me), and has a rich history.
Friends of mine have had good luck with teas and tinctures made from ephedra or "Mormon Tea", but I've never gotten a focus-inducing rush.
I've always wanted to try betel nut and coca leaf as other natural alternatives, but am unable to do so due to USA narcotics laws. That being said, these items may be available in your part of the world, and perhaps some other users could vouch for thier efficacies. Or lack thereof.
If you do choose to quit caffeine cold-turkey, I found that the perfect time to do it is when I'm sick with a cold or the flu...I'm miserable anyway, so I don't notice the withdrawal headache as much.