Almost ruined our DC trip with two out-of-state visiting relatives by trying to find metro parking mid-day. For those who are looking, your best bet is to head over to the West Falls Church metro station, where there is an overflow parking lot with 200+ spaces just beside the parking garage (same cost $4.50/day). Both Vienna & West Falls Church's metro parking lots and garages were full at 11:00 a.m. on a Thursday. The overflow lot was about 1/2 full.

If you're headed to DC from Northern Virginia mid-day, head for the West Falls Church station.

That was my experience, too - good hint for those heading to D.C.

The Huntington Metro parking lot also almost always has parking available mid-day, and is more convenient for people staying in the Alexandria area. Huntington is on the Yellow Line. (There are two Huntington parking entrances, though they are connected. I'm thinking mostly of the entrance off North Kings' Highway.)

Thanks for the tip. I was planning to use Vienna because WFC requires the SmarTrip to pay the fee - is that correct? Vienna accepts credit cards and since I'm a visitor using it for just one day, figured it wasn't worth getting the SmarTrip. I'd be arriving on a Friday morning around 7:30am.

"I was planning to use Vienna because WFC requires the SmarTrip to pay the fee - is that correct?"

This is still correct according to the WMATA website.