Pre-emptive bids are easy to make, but to make a good one is difficult as it depends on many factors.

The principle is to be disruptive to the opponents and explicit to your partner.

Table position is important in position 3 when partner has passed you do not have to worry about missing game and you can be very disruptive. In position 2 you need to be constructive as your side may have game. In position 1 the odds are that the opposition will have game so your pre-empt should be agressive.

Here is a great EBU article from Brian Senior

For a lot more detail this series by Andrew Gumperz is very good.

Most Damaging Pre-empts 1

Most Damaging Pre-empts 2

Most Damaging Pre-empts 3

Most Damaging Pre-empts 4

Most Damaging Pre-empts 5

Most Damaging Pre-empts 6

Most Damaging Pre-empts 7

The principles are also valid to weak 2s and weak jump overcalls.

As always it is important to agree with your partner the meaning of your bids at positions 1, 2 and 3.