Steve
With dew, the primary mirror will usually be OK but the secondary and eyepieces can fog up very quickly short term you can take a hairdryer with you if you have power and use it to defog (with the secondaty take out the eyepiece and blow doen the focuser. A bit clumsy and the neighbours may wonder what is going on but works.
Long term a dew constroller and set of straps works best. You can get heaters for the secondary as well. Check out
http://www.bintel.com.au/Accessories...egorylist.aspx
Malcolm