Many things can happen during a competition, and you don't need more worries :) !
At least, if you prepare it well, you will not get bad surprises about missing tacks or papers...
Here's my check List (showjumping), it's a very simple one. :
Horse Tack :
Head collar & Lead rope
Bridle and Bit
Extra pair of reins
Saddle soap/leather cleaner
Horse Care :
Feed & bucket
Hay & Hay net
For the rider :
Show boots and pants
Whip and spurs
Horse insurance card
As I said, it is a very simple checklist ! Feel free to complete it :)
I hope it's going to help you :)
I make sure I bring extra everything
I'd add to Philippa's: polo wraps, saddle pad, blanket and sheet is he wears one, grooming Kit and first aid kit, just in case :)
Well, first of all, don't forgive your horse :p
Saddle, bridle, halter, lead rope, extra pair of reins, breast collar!