A) ASAP get it checked out by MW/GP, esp after 20wks as it could be pre-eclampsia. Headaches are very common in 2nd and 3rd trimester (I got loads in early 2nd, but still got them checked out). If they feel it is 'just' headaches they may be able to give you something stronger than paracetamol, though if you're coping with them, they won't.
B) if it is 'just' headaches then the following might help:
1 Drink plenty of fluids - dehydration in pg is very common. Especially overnight, so if LO's waking you up, or your bladder is, then drink half a glass of water.
2 Food - 'have a snack before bed (banana, toast etc). Take some biscuits to bed, too and if you're waking up early, have a snack and go back to sleep for a bit. Or at least eat asap when you wake up. A dip in blood sugars could be causing headaches.
3 Food - you could have become sensitised to something. If you have any IBS like condition, or migraines, then it's probably the same triggers. Look out especially for citrus fruits, raw tomatoes, blue cheeses. Otherwise, just see if you notice any patterns. If you have an upset stomach around the same time, then there's something dietary happening (headaches to go with IBS like stomach complaints are very common).
4 Caffeine - if your earlier in pg and/or have given up/cut down on caffeine then this could be causing headaches, so just be careful. Cut back 'slowly'
5 Caffeine - if you're 'caffeine' free then caffeine CAN help with heaches. It won't work if you're a 6 cups a day caffine pre-pg drinker, but, often a cup of tea will help. It acts as a relaxant. This is why paracetamol plus works well for some, and not for others. BUT, be careful how much caffeine you consume - limit guidance
6 Like saltyair said, having a cool head can help. Use icepacks on your neck, keep your windows open at night, have a cool shower, washing your hair before bed etc. You can get 4head quick-strips (boots do their own, as do many others) if the menthol in the sticks makes your eyes water. I prefer these at home as they seem to last longer than the sticks. There is a cooling stick that doesnt have menthol in it, but I can't think of the name right now.
7 Some people likepiprabbit like having a heat pad on their necks/backs for tension relief...so might be worth looking in to. Again, pillows, strips etc can all be got from most pharmacies. I like a cool head, but a hot water bottle on my back.
8 Make sure you get some form of exercise during the day. Even a walk around the block will do.
9 De-stress. Cut out as much stress as you can when you have a headache, esp when pg as it's very draining to deal with both. Also try breathign techniques to helo your through it.
10 Sleep - make sure you're not tired.
11 Darkness. Just spending some time (90mins) in subdued lighting/darkness can help a lot. If this does work very well, then get back to your GP as it could mean you're having migraines, not just headaches. Again, common in pg, but an absolute bastard...
I'm a migraineur FWIW and these have helped me cope. Luckily not had many migraines during pg, but have had headaches.
Fingers crossed for you.