Toyota Repair: Carbon build-up causing problem starting every 12,000 miles, camery, valve assembly

Question
I have a 1995 Camery LE 4 cylinder which I bought used in 1998 with less than 30,000 miles.  At 55,000 miles, a problem starting the car developed (starts okay but would immediately die; idle control not kicking in).  Turned out to be carbon build-up in the valve assembly idle unit (which was replaced under warrenty).  Since then, about every 12,000 miles, it has to be cleaned out and on my last visit, was replaced (now has 120,000+ miles).  Any idea why this is happening?  Is there anything I can do to prevent it?  

Answer
This is mainly due to crank case vapors getting into the intake system which is normal, is the oil being changed at least every 5,000 miles? Contaminated oil can cause the carbon to build up, also an oil level that is too high, try using synthetic oil or a different brand of oil.
Remove the valve cover and check for excessive sludge in the engine, this can cause the problem also.