I try to write down their drivers licence number on the bill of sale (in case there is ever a problem or you are handed counterfeit money).
If selling, I always write something like "I, john doe am selling a yamaha zzr 250 VIN 789456123789 as is, where is to jane doe for $4000. $4000 cash was received and no further money is owing. From this point forward, Jane Doe is responsible for the bike (including insurance if it is required). Signed and dated John Doe, Jane Doe (DL#)."
If buying, I add a sentence like "The seller certifies that they do not believe there to be any liens on the property and they have no knowledge of any reason the property cannot be legally transferred (ie. it's not stolen)." I have been lucky at avoiding stolen property so far and I have no idea if this sentence would help me out legally if I got caught with stolen property, but I can't see it hurting.
Make sure you keep the plate half of the ownership.
Good luck