Which type of cells are you using?
Recently I have transfected (with Lipofect-Amine) PC12 cells which
five days later still express at a high level. I have also
transfected both 3DO(mouse T-cell hybridoma) and Jurkat which did not
yield comparable results. They both exibited minimal fluorescence
after day two and then a steady decrease(as you experienced) These
cells were transfected with electroporation. Further studies will be
done to test DEAE-Dextran for a better efficiency. Currently, I am
using the pGreen-lantern vector from Gibco for the transfection into
the PC12 cells and am getting close to 40% efficiency in transient,
which is stable for at least five days.