It's a process, not a concoction for me. I cut in the early morning; I cut the stems a TAD longer than I really want them, then cut again at an angle; I bring a bucket with some water in it with me when I go out for cut flowers so that they never go without; I place the arrangement in a shady location in a cool part of the house; and I change the water very frequently. I have not discovered any water additive that will make cut flowers last any longer than that.

I agree with rhizo. That's how I cut my flowers too. When you cut flowers and don't put them immediately in water the cut end starts to dry and close in and the stem can't take up as much water. Cutting early in the morning is important because the flower is plump with moisture after the cooler night. Recutting the stem allows more water too.

The biggest mistake most people make is leaving leaves on that will be below the water. A leaf in water starts to rot and then the flower is taking up water that will make it rot too.

You may read things like add bleach or sugar or pop but nothing beats fresh clean water so change the water often and recut the stems about 1/4 inch each time you do.

You will discover that some flowers last longer than others as cut flowers

See how easy? :-)

I've been wrapping the stems in a wet towel. Is that enough moisture to travel with them?

Funny thing re some cut flowers last longer than others...there is one color, in my Cut and Come Again plants, that lasts forever and another that dies within a day or two

For the most benefit, a cut flower stem needs to be submerged in water immediately. Submerged. It truly makes a huge difference.

To transport cut flowers in the car, I get a box with dividers from a liquor store which holds a mayonnaise-type jar with flowers and water in it without spilling.

For the house, I agree with others. I put them immediately into a jar of water in the garden, and recut stems before plunging them into vase water in the house. No leaves below water and change water every day or so.