pagerank is a function of incoming links,

there is ONLY one way to increase pagerank

increase the number of incoming links to your site

[your own site structure can effect the way pr is distributed through your site]

I found on internet that the Page Rank algorithm is calculated as follows:
PR (A) = (1-d) + d (PR (T1)/C (T1) + ... + PR (Tn)/C (Tn))
where

* PR (A) is the Page Rank of page A,
* PR (Ti) is the Page Rank of pages Ti which link to page A,
* C (Ti) is the number of outbound links on page Ti and
* d is a damping factor which can be set between 0 and 1

If I properly understood that means that the more I create inbound and outbound links the merrier the PR increases.
Is that sufficient for increasing the PR?

my website is 8 years old...I got links...it is in google index (1000-6000 pages)...it is good described, healthy organized and every week updated...but for all that the PR didn't increase since 2 years and still hanging with a value of 3 out!...I've also tried cross linking but it didn't work.
does someone know a good trustful link exchange program that can increase the PR?

whoah, hang on...
Pagerank is only one part of the Google ranking algorithm.
That algorithm, the one that actually delivers the SERPS, puts together the results of many scores and "algorithms" to list various webpages in order.
The reason PR is important is that the Pagerank score is made public knowledge for every page. We know about it. However, don't be fooled into thinking that it's all you need to work on.
A couple of points: in the Pagerank equation, outbound links are a divisor- they don't increase Pagerank at all. What that means is, if a page links to you that has fifty other links, it's less valuable than a page that links to you, and has no other links.
So, pages with lots of outbounds don't pass as much Pagerank.

However, if you have good, relevant outbounds on your page, this can help SERPS. This has nothing to do with Pagerank. It's a different part of the Google algo, a score that you will never see.

The same is true for trust. This also is not Pagerank (some call this factor "trustrank"). Google will never tell you your trustrank. But it goes into the final mix along with Pagerank, relevant outbounds, and many other factors.

So, pay some attention to Pagerank. But pay attention to all the other factors that are known to affect SERPS as well. Unlike pagegrank, you will not know exactly what your score is for all those things, but you still need to work on them.