Nelly,
I'm in no way an expert, but hope this helps a little
My LB is 7 months old, and I express all his feeds for during the work day, and then feed him mornings, evenings and weekends.
I express during the feed - he gets one boob, and I express simultaneously on the other side, alternating each time - then I usually get anywhere between 50-100mls.
Then I also express once a day (afternoon) at work - from both sides - and I can get between 150-225mls.
I give him all his feeds over the weekend, and he still gets enough milk for his mid-morning feed (the one I miss at work).
I struggled a while ago and asked advice, and got some really nice tips/links - here is the thread
www.moomie.co.za/forum/5-your-baby/16672...sing-breastmilk.html
What seemed to help for me in the end was lots of jungle berry juice (or coke if I didn't have).
Also, I've been trying to keep on expressing for a few minutes after the milk stops flowing - aparrently this help due to the 'increase in demand'
Lastly, I express as much as I can over the weekend, so that I'm a bottle (or 2) ahead by monday - helps to take the preasure off during the week