It's a bit tricky to get your mods to stay with this method, what I did was move my existing Minecraft.jar file to C:\~~~\AppData\Roaming\.minecraft\versions\1.5.2 and rename it 1.5.2.jar. Then you have to keep the jar file open with Jzip (or whatever zip program you use) and click play in the launcher. because the jar file is open the launcher won't rewrite it, and you'll have your mods.
This is somewhat of an annoyance and unnessicary as you can still use the old launcher for 1.5.2, and play 1.6 via the new launcher.