I am not too good with feeding my plants but I know all clemetis like a handful of lime around March time and I sometimes give them some bonemeal or fish, blood and bone late Summer - early Autumn. Mine seem to thrive on that. Good luck.

I use Miracle Grow.
Follow the directions and the plant will be wonderful.
(I also use it on ALL plants in my garden)

I use Phostrogen on all my plants, it's considerably cheaper that Miraclegrow.

Erm, I disagree with the 'handful of lime' advice above and would not recommend it. If your clematis is in the ground, rake or turn into the soil around the base a handful of Vitax Q4, Growmore or similar general purpose fertiliser in April. Repeat six weeks later if you want to (I don't) and that's it. If in pots, once a month or fortnightly with Miracle Gro general purpose from April through to mid June.