Try going for a 2R to 1C/I, in some cities I've had to go up to a 3 to 1 ratio to stop getting the "Not enough workers" message. Also, be sure to throw down some schools or have you sims bused to other cities for education(put down SCHOOL bus stops from another city w/in the city you're working on) if available, so businesses want smart sims and won't take uneducated sims. Also, it doesn't hurt to go w/ high density roads, you can get more sims stuffed into an area that way.
If you don't understand my 2 to 1 description think of a big square, divide that into 4 smaller squares, 2 of those smaller squares should be Residential while the other 2 should be 1 of Industrial and 1 of Commercial. I find getting the right balance to be rather annoying at times, especially when you start throwing in the different wealth levels that effect R and C, thankfully it's just education that really effects I. Although wealthier sims do seem to enjoy having "better"(more advanced) Industrial jobs... Hmm the RCI interaction is certainly complex...