My friend did this last year [actually a whole bunch of us got together and did all the main pokemon girls]. May's costume is actually pretty easy: red polo with dark blue detailing near the collar, white skirt, dark blue or black underarmor shorts, optional yellow fanny pack, red bandanna with pokeball image, white gloves with blue fingers, black socks, red and black sneakers [I suggest converse 'create your own'].
If you want visuals, I have pictures on my profile, message or post here if you want more details as to how we did anything.
Otakon 2011 Lineup
Friday: Paula [Earthbound] and Lina Inverse [Slayers]
Saturday: Lina Inverse and Twilight Sparkle gijinka [MLP:FIM]