As a geek, I have so many whiteboards around me - they come in so handy!
As many people know, if you leave the ink on for a while, it is possible for it to dry out and require a lot of elbow grease in order remove.
Instead, a little hint - just get out your dry wipe pen and go over the top of the text!
This techniques add another layer of the chemical carrier and allow you to remove the original ink as if you had only just written it!
I went to a client that hadn't used their meeting room in a while and they had this problem - They did not know this technique and I could see the bloke trying to scrub really hard! I hope I didn't come across big headed but this is something that a lot of people just don't realise you can do!
Hope it helps others!