DVDSP should handle this automatically for you - it's in the prefs. If not, you can place a marker manually and in the property inspector you can set it as a layer break marker.
The rules are that layer 0 must be bigger than layer 1, but it may not be obvious where in your track this needs to be, depending on your disc structure.
You could also try changing the DVD settings from OTP to PTP and see if that helps. This will introduce a slightly longer pause as the layer switch takes place as the laser has to physically reposition back at the inner part of the disc from being on the outer edge, but it may get you out of a fix. PTP is fine if you are using lots of small clips on your disc... in fact, I use it out of preference in that situation. OTP is really used for when you have a single long track, or at least a very obviously main feature on a disc. In this case the layer switch happens at the outer edge and the laser starts to track back in towards the centre of the disc.
Finally, in the DVDSP preferences for the track, make sure that you have checked 'fix invalid markers on build'.