Lessons. And practice. And more lessons. Then more practice.
I got lessons off an England international caster when I lived down south, took me about three months to get from my initial paltry 80 odd yards to 200 yards then another month to get to 200 metres, half a dozen lessons in all plus ending up a regular down Steyning with the Sussex casting club (you don't have to take it that seriously but it is nice to go along to a casting day and get a proper measure)
Get in touch with Bob Collins (arc on here) or Peter Thain (google him and his facebook page should come up) and they will see you right. Bog standard kit is good for 150 - 180 yards, after this having a decent mid range rod will get you up to the 200 yard line...