These are general tips I've written to help others as the presale is ongoing and the general sale starts March 13th 7am GMT.
1)Patience is a must. It may take a couple of hours to buy a ticket. You'll have to be checking your time left in the queue as once through you only have 2 minutes to decide if you want the tickets offered.
2)Don't be too fussy if the seats are not ideally placed, if you try again for better tickets they may be sold out the next time you have come to the front of the queue.
3)If the site goes down, like it did on the first day of the presale, don't panic and try again. If using your presale code it will still work until you have actually completed an order for tickets.
4)Don't open more than 1 browser and try to buy tickets for different shows. You'll be queuing for a long time and then kicked out the queue just as you think its coming to an end.
5)Certainly in the presale, go for the later dates as they were added as earlier dates were sold out.
6)Certainly for Friday 13th March, which is the start of the main sale you'll have to be online and at the ready at 7am (GMT), no later. I did read in some fan forums that some people thought they bought tickets earlier than 7am. Some stated arund 6.40am. Other people said this was not the case, but I guess it can't hurt to try a bit earlier.
7) Don't fret about Friday 13th be an unlucky date. No matter you what might go wrong. If your online at 7 am then your in with as good a chance as anyone of getting a ticket. If things go wrong, keep trying!
See also Michael Jackson Press Conference at O2 Arena and general Michael Jackson ticket news.