I don't tweet as often as I should. I used to be 10 times more active than I am now, but time is not on my side. I tweet about 2-3 times per week. I usually tweet about interesting articles I've found online or about my own blog posts. I really don't assess the value at this time but there are plenty of Twitter stats sites around for stuff like that when needed.
How often do I tweet? About a dozen times a day. On behalf of my company... well, tweets for my company website, blog, or sales pages are only a very small percentage of the total tweets. All other tweets are, (surprising to some) spreading information that other companies have to share. If it is good stuff, share it. That is the beauty of social media. Assess the value of social media... there are analytics tools that can track all of your stats, traffic, ROI, and whatnot but to measure the actual VALUE... well, I have to ask, How do you measure influence? Tips and tricks.... This is my opinion but when it comes to social media, I don't think any "tricks" will ever hold much weight for very long. Biggest tip