1- If you are not on the route anymore, you have 3 choices:
a) try to get back to the route by yourself by stopping and looking at the screen (i guess this one you won't like it )
b) press F3 key while navigating and it will calculate a new route for yourself from your gps position (take care if you are using this solution to always delete the past stop points you don't need anymore because route recalculation can by mistake return you back to an old stop point if not far away)
c) in S&T 2008 and 2009, there are an auto route recalculation feature that avoid to press the F3 key and will always recalculate a route. (in S&T 2008, this is mandatory, and in 2009 version, it is a real option that can be disabled. For myself, i am always keeping this option disable because it can mess things with your planned route and i prefer to press F3 key only when it is really needed)
Comment: if you want to use the F3 route recalculation option, i suggest you install S&T Keys 2007 (you can see a link in my signature), it will add a hotkey (DEL) that will always clean all previous stop points you went by, so in case you need to press the F3 key later, it is very usefull to avoid you going back to a previous stop point. There is no manual for the old 2007 version, but you may take a look at the 2009 version of S&T Keys and the online manual (web page) is mostly very similar to the 2007 version...