A first step could be to lower the CO2 content (my current favourite would be to release engineered bacteria into the atmosphere that can bind the Cs and after death collect it on the ground), and enhance the albedo (so more sunlight is reflected away).
In the longterm we will need climate control anyway since Earth's climate is somewhat unstable (global warming is just as bad as a new ice age, which, extrapolating from the past, would be about due by now, if not for mankinds influence). Testing it on a planet we are not critically dependent on should be a better strategy.
Geologically, Venus is very different from Earth due to the lack of water (which enables plate tectonics and all its implications). Some changes we can make would even influence the geology. (Hm, I wonder what would happen if we dumped enough water meteroids.. )
All of this of course premises a lack of preexisting life with planetary ownership rights and no natural reserve rules (which we kind of have at the moment - http://planetaryprotection.nasa.gov/about).