I am planning my 1st trip ever to DC & would love to hear the locals advice on the best time to visit.  Thank you in advance!

If you are talking weather wise, the best time is FALL or SPRING. Winter gets too cold and summer too hot. If you are asking traffic wise, its pretty bad all year round. As long as you stay in popular spots like Tyson's, Dupont, Georgetown, you should be able to do both, have fun, and see clients without much difficulty. If you stay in Georgetown or Dupont, you don't need a car, while if you stay in Tyson's (VA) you are better off renting one so you are not stuck in hotel all day long. Public transport really sucks in VA. Hotel prices are pretty high during workdays, and are lower on Fridays and Saturdays.

Hope that helps...

sightseeing & tourist stuff, avoid the summer months when a zillion families are here touring, also avoid April when schools across the country are on Spring break & flooding the place.  October & March are good, weather unlikely to be harsh & tourists at a low ebb.  If you're looking for business, it's a bit less clear.  Much of the govt. (and associated contractors & conventions) disappear from mid-July to the end of August.  Hotel prices are highest in the summer & on weekdays the rest of the year, November (except for Thanksgiving weekend) & February probably have the lowest rates.  
The weather here is not usually too harsh in the winter (the blizzard of 2 years ago being the exception) but the summers are always very hot & humid.  this is more of a southern city than most folks realize.
Hope this helps.

I know I didn't ask the question, at least not out loud/on the board.  But I'm in a similar boat and on my first tour here in DC and every tidbit of info helps.

but Winter may be the best for business. Summer is often slow and any time close to holidays can be fiffy. I have heard from a very well respected provider her best days can be right around holidays but others generally say the opposite.
I would agree with other posters on the hotel rates but the internet is always your friend..very attractive rates do pop up no matter what day and time of year.
As to locations, there are three distinct markets, DC(DuPont Circle & Foggy Bottom are very popular) Northern VA (Tysons Corner and Cyrstal Citey/Rossyln two preffered choices) and BWI Airport area. Because of the area's size and grim traffic (we are now No. 1 in congestion overtaking LA..oh finally for joy!), many girls stay a few days in diferent spots.
Im told the flaky client issue(e.g. drunks, NCNS calls at all hours for example) is much more prevelent in DC than NoVa but in NoVa business can be slow on weekends.
Hope this helps..Best wishes to you all..