ASCE 7 allows for two or more transient live loads to be reduced by 0.75, with no reduction in live load. For example D+L+S can be considered as D+0.75(L+S). It is important to consider the effects of load duration when using these combinations if you are designing with timber. What you'll find is that members supporting two floors and roof usually don't need to be designed for roof loads at all, if the roof load is relatively light. This may seem counterintuitive, but is perfectly consistent with the stochastic basis of the uniform loads in ASCE 7 and provides for more uniformity between ASD and LRDF design.
For multiple stories of live load ASCE 7 does not provide clear guidance, however HUD has published some intresting 'Residential Construction Design Guide' and 'Structural Design Loads for One- and Two-Family Dwellings' which provide intresting perspective on load combinations and reductions. It provides a floor LL reduction equation developed for residential floors. It also recommends greater load combination reductions and presents the load combination of D+L1+0.7(L2+L3+...), where L1 is the story for which members are being designed and L2, L3 are the live loads imposed by floors above.