1. Contravention loophole
The contravention did not occur: for example the signs or lines were wrong.
2. Know the rulebook
The relevant Traffic Regulation Order (which allows councils to set restrictions) is invalid. For example, if the stretch of road on which you parked wasn't included in the original order. Requires a fair amount of digging for this to work.
3. Check the paperwork
The council hasn't done its paperwork properly: for example, because there's a lack of witness statements from the enforcement officer.
4. No challenge
The council did not respond to a challenge or responded too late.
5. Car failure
Your car broke down. Most councils will rip up a parking ticket if it's a sudden, unexpected failure, but not as the result of poor maintenance.