I've noticed that there's an awful lot of theory discussion on how to play as town, but very little on scumplay. And I think this is a particularly useful subject for discussion. Most of the 'easy' town wins I've seen could have been close, if even one scum had fakeclaimed competently. So, here goes...
1. READ THE RULES...
and don't contradict them. An obvious one, but one that catches a few new/careless scum players. In Choose Carefully Mafia, SocioPath claimed a scumgroup-specific cop, when Bird1111's rules made clear that power roles had to choose which scumgroup to target. Sometimes rulesets will also include things like the total number of power roles, or the order for action resolution.
2. KNOW THE GAMESTATE
Make sure you're clear on how many nights there have been, and who else has claimed what. Fit your claim around it. If you an RB has died, you can claim to have been RBed while he was still alive. If there's a claimed watcher, don't claim to have targeted his watchee. You have a tradeoff to make here- if you fit your claim entirely around publically available information, it's likely to look fake- but you don't want to give the town too much setup info here either.
3. KNOW WHAT YOU'RE TRYING TO ACHIEVE
If you're the last scum, it doesn't matter if you're being lynched today or tomorrow. You hve to survive to endgame, so your claim should be geared to allowing that. Weak investigative, or blocking, roles are probably best.
On other occasions, delaying your lynch even one day can make a huge difference. In Mafia 58- Ready Salted Mafia, I was tracked performing the scum's nightkill, but managed to wriggle out of the lynch by claiming to be a Watcher who had seen a townie do it. The townie got lynched, I was able to nightkill the cop that night, and instead of an easy town victory, my buddy Battle Mage got to endgame, and would have won had the townie he was pushing not gotten modkilled. One-shot claims, or claims that can be 'tested' that night, are useful for this.
4. KNOW YOUR CLAIM INSIDE OUT
Make sure you know what the flavour justification is for your ability. Make sure you know EXACTLY how it's supposed to work. If the role you're claiming has night actions, ensure that you can come up with a town-sounding justification for each choice. Oh, and claim the correct rolename- for example, it's 'Psychiatrist' not 'Psychologist,' 'Forensic Cop' not 'Forensic Scientist' and so on.
5. KNOW YOUR MOD
Different mods have different role preferences. Mr Stoofer, for instance, would never include a Paranoid Gun Owner in his games, so claiming one would likely get you caught out. Many other mods have a strong dislike of the cop + doc combination. If you're claiming cop, do some research as to whether the mod has any history of using variant sanities.
6. MANAGE YOUR SAFECLAIMS
If you have several, discuss with your partners which names seem suited to which fake roles. If you have fewer safeclaims than group members, you need to think about when to use the safeclaim. On the one hand, scum players claiming later have more information, so are likely to be able to come up with a better claim. On the other hand, if one of you comes up with a terrible claim, then town are likely to conclude that you don't have any... which will make life a lot easier for your buddy down the road.
7. GAMBITS CAN BE EFFECTIVE
But you shouldn't be trying them if there's no need. Weigh the risk and the reward If you think you are likely to be today's lynch, then claiming an incriminating investigation on an innocent at the start of the day can put you one mislynch better off than you would otherwise be. If you're in a town that is shy to policy-lynch, then claiming miller day one can be a great way to neutralise the threat of a cop. Just don't do it if the town includes me or Xyl. Deliberately nokilling to set up a doc or roleblocker claim can work, though you need to balance the risk of leaving that additional townie alive.
8. BE UNPREDICTABLE
In Pirates vs Ninjas, TheStatusQuo as town was able to see through Guardian's fake tracker claim easily, because 'Guardian always claims tracker when scum.' For obvious reasons, letting town get this kind of read on you is a bad idea.
9. AVOID 'HUGELY CONVENIENT' CLAIMS
Some claims have just lost all credibility. In particular, Super Saint and Bulletproof. If you claim SS, then town will just try to get another suspect to hammer you.
10. LIE THE ABSOLUTE MINIMUM POSSIBLE
The more different lies you employ, the more chance you have of being called on one of them. Keep it simple.
11. KNOW YOUR OWN ROLE
If you're a scum roleblocker or doc, then your ability is trackable. This can lend credibility to power role claims.
12. FIT YOUR BEHAVIOUR TO YOUR CLAIM
One of the best examples I've seen of this was LlamaFluff in Mafia 50- Mafia.gif. Setting up a psychiatrist claim, LF seemed to fixate on the SK far more than seemed rational given the game state, and this was a big part of why I bought his claim. If you're claiming cop, and you attacked a player early on then changed your mind, an innocent investigation overnight will seem very plausible. If you're claiming doc, then a player who you've been saying is town all game makes sense as a protectee. If you're claiming RB, the perfect target is someone of whom you've expressed suspicion, and who's claimed vanilla.