I just bought aquarium plants from the pet shop. My question is how do i preserve them from dying so they can be used for next days use?

They shouldn't be dying that fast anyhow. Whenever I get new plants (and it hasn't happened lately, since my fish eat them anyway, so its a waste of money ) I had them sit in a bucket of water and PP to desinfect and kill any possible nasties over night.
YOurs should be doing fine overnight in a bucket of water. Do get them out of the plastic bag they came in - that thing fouls the plants fast...

do you have good lighting system. buy some elodea densa