Hi, take a look at this link for a general idea:
Remember that the divalent cations in the annexin buffer can cause cell clumping, so you need to be careful.
I do not exactly know what your application is, but if you are only looking for early apoptosis (AnV+/PI-), go ahead and trypsinize your cells (monitor closely, and take care not to over-trypsinize). You'll get an increase in AnnV-/PI+ cells, but thats usually less than 5%; and at least for early apoptosis (driven by expt treatment), the benefits and convenience outweigh the risk of clumps and death caused by the slow detachment/mechanical disruption.
We have successfully used this for lung and kidney primary epithelial cells, and cell lines including MDCK.
Also, use an alternative assay such as Casp3 activation/LDH release (can be done in the same well: sups vs cells) for additional corroboration.