When it comes to roleplaying games (RPGs) the above title may confuse some folks, on two separate levels. First, there’s the question Why should I make my players’ lives easier? Second, there’s the question Why should I not let them know that I’m doing it? Fair questions, but: there can be valid reasons why sometimes you should and should not, respectively. For example: it’s one thing when player characters (PCs) do something dumb on their own, but one of the problems with any game that incorporates random chance is that sometimes the dice will conspire against both the PCs AND the gamemaster (GM). Even GMs who are utterly without mercy* might be inclined to put a thumb on the scale if their own campaign’s neck is right there on the chopping block, too.
As to why you shouldn’t sometimes let your players know that you’re giving them a hand? Well, pride’s a thing. Players prefer it when they get themselves out of a mess by being awesome, and they’re there to have fun too, right? Besides, you don’t want to bail them out every time they’re in a jam; they might lean on that too much. So having them raise a little bit of a sweat is a good idea.
Now that we’ve gotten that out of the way… I assume that everybody reading this knows that they need a GM screen, right? I absolutely refuse to waste column space here by justifying why you should have a GM screen. Particularly in this context. Just get one, and roll your dice behind it.
Moving along… the first thing that you want to remember is: adverse conditions affect everybody. Particularly environmental ones. To give just one example: a good, driving rainstorm can justify anything from why the enemy isn’t using their bows (the strings got wet) to why the encampment has a gap in the sentry line (who would attack in the middle of a hurricane?). If you give the PCs enough ‘advance warning’ (all hail the power of “make an Intelligence/Luck/Perception check and tell me what you rolled”) they can mitigate the adverse effects on their end and snicker as it becomes clear that the enemy has not.
A related trick is this: not every enemy is a Terminator. A lot of them, in fact, can be dumb, unskilled, inexperienced, unlucky, or simply in the wrong place at the wrong time. People who haven’t been in combat lately can become complacent. People who have been in combat lately can become jittery. Either type can be superstitious or in possession of bad information that caused them to make bad choices. And the only thing you need to justify all of that retroactively is to have the PCs find a blood-stained note that ordered the bad guys to do something that turned out to be really, really stupid. If it’s a stupid (and exasperating) thing that your own PCs themselves do, all the better. Then it’s also a subtle, conscience-clearing, hint.
And then there’s the good, old-fashioned deus ex machina, which has been getting people out of plot holes since the days of Ancient Greece. The trick with this one is to do the prep work ahead of time: basically, pick what the Get Out Of Trouble Free manifestation is going to be, and then work in suitably obscure and vague omens. For example: using an earthquake to, out of the blue, bust the PCs out of jail is kind of obvious. But setting it up ahead of time that the campaign area is prone to earthquakes — maybe giving the players a small one to roll dice at — and the players will be disinclined to quibble when the Big One hits (and manages to save their hide in the process). And if the players never need it? Don’t run a scenario involving the Big One.
And, if all else fails, involve a cow somehow. If you can just make it surreal enough, your players will uneasily assume that this was your plan all along and you’re not just making it up as you go along. Even if you have been, and they just don’t know it… but that’s a different article altogether.
*I don’t actually recommend that as a steady diet. The group dynamic involved between an antagonistic GM and his players can be fragile, and if it collapses it can collapse rather nastily. Most of us don’t have so many friends that we can really afford to lose one, just over what happened at the gaming table last week.