I've used both the aspirin and sugar at the same time. One teaspoon's enough and one aspirin in a vase of water. Changing the water every other day completely seems to help too (remember to add the sugar and aspirin again!) as does keeping them in a cool room rather than hot.

Also, try submerging them completely in a sink of cold water for an hour or so before putting them in a vase. This helps them absorb water not only through the bottom of the stem but the leaves as well and gives them a good well watered start.