I used to get this alot from spending lots of time in the water swimming, surfing, sailing, diving etc.
SwimEar is good for on-going maintenance whilst on holiday for instance. This as well as always washing out your ears in the shower when you've been in the water and making sure they dry-out clean.
I've had my ears syringed before. This may or may not be needed. I'd go and see your doc first for him to have a look and recommend it if necessary.
Whatever you do don't let it get too bad as it's really painful and distracting if it all swells up and you get a mid-ear infection. Then you'll need antibiotics as someone else mentioned earlier.
Whatever you do ignore any suggestions involving sharp implements, white-spirits, small fires or explosive devices etc. though I'm sure we all realise that these are tongue in cheek suggestions (at least I hope so!).