I've done this also, always with a "group" (multi-op contest situation, such as the 160m contest) to make it easier.
We tried a lot of stuff. A BIG latex helium balloon will lift a very long wire straight up vertically, but as stated, only with ZERO wind. The tiniest puff of wind blows it far off course and a stiff breeze can blow it down almost to the ground.
What we found was a long string of smaller helium balloons works better. Never tried to calculate exactly why,
this was all just trial-and-error, with quite a few errors. Plus it's fun filling up helium balloons, as you can take a lungful and then talk funny for a minute or so.
But what seemed to work well was about 6-8-10 smallish balloons (maybe 12" or so each), tied every 10-20 feet along the wire, so the "lowest" one was just out of reach, and the "highest" one was at the very top. Seemed less effected by the wind, and the balloons are cheaper.