a friend told me that we need to increase of backlinks by simulating a natural gradual increase, but how often? a day ? a week?? or ...? thanks
I would suspect that if your site was constantly getting new back links at whatever rate for a period of time, and then started to not I would believe your site is becoming less valued than it was.
I work on it daily also. Not all of your links will be found/counted on the same day, some may take weeks before their found.
Too fast is probably not good. Especially at Yahoo! for some reason. I have experienced that several times. Whenever I did lots of link exchange, I got lower SERP at Yahoo! But then you should increase backlinks at any time. Everyday is ok as long as you don't do 100 links. If you are not into SEO for yahoo! I guess you wouldn't have to worry so much, but even Google doesn't like a sudden increase of links. Similar topic was disussed somewhere in this forum, but in my opinion, I think 10 links per day is the best.