Little help please reddit

Looming this Saturday is my 1st photography "gig".

My sisters boyfriend is involved in a local horse riding club and they have their 1st hunt of the year and the photographer they had booked has canceled so I have been asked to fill the void.

Can any pros out there give me any last minute tips?

I have a 40d, 4g and 8g cards, three battery's, 17-85mm, 28mm, 50mm and a 70-200mm (non is). A friend has agreed to lend me his 20d (and 4g card) so I dont have to chop and change as much (and il have a back up body just encase) a mono pod for wandering round and a tripod if I can find time to set it up.

I want to try shoot everything in raw (just so I have space to try hide any fuck ups) so Im thinking I might try pick up another memory card (4or8 depending on how much the local store will try to screw me)

So, any last minute tips? Pit falls as to how this could all go horribly wrong? Thanks in advance.