Been considering taking the plunge for a while now, and thus been trying to know the causes of so many failures and so far I can share the following:

Lifestyle: Being a salaried employee has taught so many of us to live some lifestyles which maybe incompatible with a start up.
For a start you may have to cut back on some non essential expenses until the business picks up. E.g In employment for those who drink you can drink every so often which in business may not be sustainable. You may have to ride a matatu for a while and times you may need to carry some sandwiches from home for lunch to spare every penny possible.
Patience: The late Nelson Muguku said that when he started his Chicken rearing business it took him three years to be able to start keeping any money in his pocket, and like they say the rest is history. I.e whatever little he made went into the business for the 1st 3 years.
Saving Culture: very important to always put something aside for a rainy day, especially knowing that no matter how Rosy things may look now the business will reach maturity and stagnate or start going down, cheaper chinese alternatives will come etc etc.
Humility: Accept to be squeezed in that matatu knowing very well you used to ride your own Benz, and when your former partying friends come knocking to go to the parties be bold enough to say I am broke.Some simple and despised businesses might be where the money is to be found so don't be ashamed to switch from the prestigious job to a fruit and or chicken seller. A certain thread here in Wazua had one member do some math which saw a chicken seller earning a Profit of Kshs 510,000.00 per month which compared to the average salary as per a certain newspaper article (KShs 31-35K) leaves one without words. Yet with a salary of 35k when you go to buy the chicken the chicken seller calls you "mdosi ama Boss!"
Those r the few I can think of now.