Duct tape - especially for 5Ks - is a godsend. When the RD realized I had done it you could have knocked him over with a feather.
We had the same problem with a rainy race day. Fortunately, I put down the start/finish lines the night before...the mile splits were substituted for a VERY clear course map telling the volunteers where the first, second and third mile splits were.
I've painted marks at the side of the road to note mile splits, only to have 90% of the participants not realize the mark existed. A straight tape line gives a clear message - 'you have passed a split.' Especially helpful when dead-head course volunteers don't read off splits, etc.