Hi Akram,
The basics of the MM system (as I've learned from Dale - I'm sure he'll post his "book length version" later :-)
Each position should not be opened with more than 1.875% of margin.
So if you had an account with $10,000, then each position should get $187.50 margin or less. When you are opening the position if 18 lots puts you at 178 and 19 lots put you at 187.51, then you *ONLY* put the order for 18 lots.
From there you can have about 10-12 positions open simultaneously and be "very safe" from money management. I think Dale's rules are no more than 30% of your account margined at any time. So theoretically that is 16 simultaneous positions open, but I prefer to stay a little below that for two reasons:
1) This gives you a little extra cushion in the event of a drawdown (which you will see from time to time)
2) This also leaves you with a little wiggle room for the day that AUD/NZD finally turns down! (Ok, so it gives you extra room to add some "good orders" when you see them, and AUDNZD short is one that someone has been waiting awhile to see
Other than that, always monitor your free cash and ensure that you're "comfortable" with any positions you take BEFORE placing the order.
Of course the other side of the coin on when to take profit is pretty much wide open. One thing I can say, I've never met a profit I didn't like :-)
Hope this helps.
Also, there was some discussion about how leverage ties into this. The one difference this will have is that with 200:1 leverage you can open 4 times as many lots as you can with 50:1 leverage. For example, same 10000 account, your margin is 187.50, so with 200:1 leverage you can "get more" lots with that 187.50 than you can with 50:1 leverage. Of course, again trade with your head, not over it.