What precautions do I have to take for doing this? I keep all my negs, colour & mono in plastic sleeves. What sort of temperature and humidity are ideal? Any other conditions I need to keep an eye on?
Wilhelm Imaging Research has got an interesting book talking about everything archival: Dead Link Removed
Chapter 16 talks about the environmental conditions.
Overall it's about keeping temperature low (lowest as possible, much better for color; B&W doesn't care that much), Humidity between 30-50% and dark. Also away from pollution.
I, my equipment and negatives were relegated to the basement a long time ago. Our basement is pretty dry as basements go, the humidity is between 35-55% (winter vs. summer) and the temp. between 65-70, which is pretty much the comfort range for us humans as well. Some of my negatives are more than 40 years old and are fine.
Ah but this is only because your basement had quite favorable conditions. 30-60% humidity is not common in basements at all. Even my garage stabilizes between 50-80 depending on season and daily weather.
Highly valued negatives should be relatively air sealed and frozen. A vacuum seal is not necessary but cold temperatures and the implicitly low humidity of a freezer are quite a big advantage.