Davin, what have you observed does happen? What gets dewed up? The secondary mirror? The primary mirror? Both?
Jumping ahead, it is more likely that you need to provide some heating to the secondary mirror (as well as eyepieces, telrad, finderscope etc.) There are a couple of ways of doing that with heaters, one requires dismantling and fitting a heater to the rear of the mirror (depending on the style of mirror holder) and one is by fitting a specialised heater to the secondary holder. Otherwise, next best may be the occasional blast through the focusser draw tube with a hairdryer.
The primary is an entirely different matter. We spend all our time trying to get the boundary layer absent by keeping the mirror as close to ambient temperature as we can, so few people heat their primary mirror. The approach that most adopt is one or more "tangential" fans - blowing from the side directly across the top of the primary. Another easier approach that might work is to keep the primary mirror cooling fan on (located behind it, centrally), but seal off the end in some way so the airflow passes around the mirror edge and up the tube. That can help scour the air away from the mirror surface and discourage dew.
Here is an example:-
http://www.iceinspace.com.au/63-400-0-0-1-0.html
I've done an 8" with a piece of wood fitted around the fan and the knobs.
Also, an upper dew shield may help protect the secondary mirror (just like an SCT shield). But one important thing is to tip your scope down so it doesn't point to the open sky, when you are not observing - leave it in a more horizontal position. Don't encourage that primary mirror to feel cold and attract condensation!