There's a few tricks with fitting oil seals.
First fit them the right way round! It sounds a no brainer, but you'd be suprised. Look at the seal lip and work out which side needs to hold the fluid tight, the lip with usually angle towards the fluid.
Everything spotless? grit, rust and **** will rip a seal lip in no time. Is it fitting over a shaft? is the shaft rusty or worn, no matter who you fit the seal, it'll leak if the shaft has a ring worn in it where the seal fits.
Lube up the seal lip and what it's going over. If you're passing it over the end of a shaft, I use a plastic bag oiled up and stretched over the end of the shaft, helps the seal over the sharp edges of the shaft.
If the seal has a round steel spring to tension the seal lip, you can pop the spring out and it will unscrew somewhere in the coils into a long spring, looking carefully at the ends you'll notice a male end and a female end, you can carefully trim off a few turns of the spring on the female end and screw it back together, tightens the seal up a little. I find this trick works well for motorbike fork seals and seals that have back and forth movement, rather than just round and round.
Knocking a seal in to place will be easy if the lips not twisted, before to get the hammer, carefully check the inner lip is straight and hand push it as far as it will go, then tap it up with a socket or suitable drift, make sure it fits the outer part of the seal and you're not resting on the inner part of it.