Learn your times tables off by heart up to at least 12, play darts, work behind a bar and tot up your supermarket shop as you go round with the trolley.
And I'm not being facetious. It works - constant practice.
The main tricks are a) approximating numbers; rounding up or down to get a feeling of the magnitude of the answer; b) splitting the sum into several parts and c) did I say learn your times tables?
Anything multiplied by 5 is the same as multiplying by 10 (ie add a 0 on the end) and dividing by 2 (ie halving).
Then start multiplying 2 digit numbers by other 2 digit numbers in your head, i.e. 27 times 43 is 20 x 40 plus 7 x 40 plus 20 x 3 plus 3 x 7 = 800+280+60+21. The move onto 3 digit numbers.
When you can do those, a simple 1 in 60 rule will be a walk in the park.
Cheers
Whirls