Tony Travers' mayoral voting guide

DO mark your ballot papers with crosses rather than the numbers one and two for your first and second preferences.

DON'T put your crosses in the same column, making it impossible to work out which candidate is your first and second preference.

DO mark one first preference vote - if you only mark the second preference, leaving the first column blank, it will spoil the ballot paper.

DON'T vote for the same candidate twice - although your first vote will count, your second one will not.

DO remember that no single candidate is likely to get the 50 per cent vote share needed to win on first preferences alone, so your second preference vote will count. But only the two top candidates - likely to be Boris Johnson and Ken Livingstone - will go through to the next round so it is likely only second choice votes for them will make a difference.

DO vote for different parties for each vote if you want to. You can pick different parties for Mayor and for the Assembly, or pick one party for your Assembly constituency and another for the Londonwide list. Or you can give the same party all of your votes. Filling in just the mayoral ballot paper, but leaving the others blank, will not invalidate your vote.