You are inkjecting directly into the cell cytoplasm, so detergents and high concentration of glycerol, salts etc are out. You have also to get the solution in and out of the pipette so viscosity can also play a role. Finally, be careful of proteins which change their state. When you get it over the cells at 25C, it polymerises and blocks the needle. There are different tricks for each outlined in the relavent sections below.
Cells are unable to balance intracellular pH against strong buffers.
In a perfect world cells should be microinjected with a buffer which induces the least perturbation. We have found that dilute phosphate buffers such as Dulbecos PBS or 50 mM Soduim phosphate are easy and effective. While this buffer stabalises many protein solutions, you should be aware of the problems of bacterial growth since it supports life in all forms very well. Unlike protein when microinjecting DNA and RNA be aware that the DNA has a significant buffering capacity. Finally, always inject the buffer used with marker antibodies as a control.