i had for a black lab i had over 10 years... and yes, i did lay beside her on the floor and cry like a baby, its a very hard thing to do.
i used to work as a vet-tech and i can tell you that some people waited WAY too long to put their pet out of pain. as someone else already stated, pets are very good at hiding their pain. if you think they are in too much pain, then chances are they have been in too much pain for a lot longer than you realize.
its a horrible thing to do, but in the end its the only choice sometime.
no way to prepare for it, just pet them while they are going to sleep and try to remember they will never be in pain again... then ask the doctor to leave the room while you cry.. thats how i handled it
good luck, and sorry about your upcoming loss
