What are the dos and don'ts of finding a job in Kenya?

On the whole, I agree with the above.  Kenya's job market has become increasingly difficult to break into as a foreigner for several reasons:
-There are a high number of Kenyans with degrees and masters degrees, who are unemployed.
-The government discriminates positively towards the local workforce - employers are only supposed to fill positions with foreign workers, where there is a skills shortage.  Even then, they are supposed to train a local worker to do the job, so that the foreign worker can be 'let go' at the end of the 2 year work permit cycle.
-Foreign workers expect higher pay and better living conditions - the government have fairly recently commented on this and had started to review NGO jobs, to see whether it was necessary to employ foreigners.

There are still a significant number of NGO & UN jobs available for foreigners.  They tend to set the bar quite high though, often requiring education to masters level, plus previous experience in the sector. 

There are also opportunities for volunteer work for organisations such as VSO, which can be a good place to start for those looking for a career in the NGO sector.

Probably the best place to start are sites such as 'reliefweb' and to start the process from your home country.

I think that it's unreasonable to expect a long term contract - you won't get a work permit for more than 2 years.  This may then be renewed, or not - it's by no means a certainty.

It is also not unknown for the government to review foreign NGO employees and for work permits to be cancelled and workers deported, where they feel employment of foreigners is unnecessary.

It should be noted that ANY work, paid or voluntary is illegal on a single entry visa and without a specific permit.