Bug guts are a mix of fatty acids, fats, whatever the bugs were eating and other bug parts.  Being fatty residues they are most easily removed with mild bases, so most home detergents (spic and span, Mr clean etc) do a good job especially if you pre-soak and then scrub off.  On on windows, windex or the equivalent has a little ammonia which also acts as a base to loosen up flattened bugs.  @ times over usually does the trick.

The rain-x type of pretreatments are fundamentally silicones which leave a hydrophobic surface on the glass and the pretreatment can make the splattered insects less likely to adhere.  Likewise, the wax you do the car with will also lower the surface energy of the painted parts, again making bugs less likely to stick.