Summarize:

Of the nearly 1500 species of starfish in the world, they all have one thing in common: they're slow. It can be very difficult to tell if a starfish you find is living or dead, but you can look for a few key signs that you'll be paying tribute to the beautiful animal by preserving it, not killing it.  If you come across a starfish on the beach, wait to touch it. Look at it closely. Is there any movement? Is there any bubbling from the sand underneath? If so, do it a favor by returning it into the water. Watch closely for several minutes to look for signs of life before picking it up. If the starfish is brittle and unmoving, it's dead and safe to take home for preservation and decoration. An optional step before preserving the starfish is to take some time to clean it up for display. While not essential, some collectors like to submerge the starfish in slightly soapy water and dry it thoroughly before soaking in alcohol or drying in salt.  If you want to do a pre-soak, dilute a tiny squirt of liquid detergent in several cups of water and soak the starfish to get it clean. Don't scrub or otherwise over-handle the starfish, which is fragile. Dry the starfish thoroughly in the sun, taking care to weigh down each arm of the starfish. They tend to curl up some as they dry, so it's important to flatten them gently between two plates to maintain a uniform flatness. Generally, most collectors will go straight to the alcohol soak, but do whatever seems appropriate for the specimen you've collected. When you get your starfish home, soak it in enough isopropyl rubbing alcohol to cover the starfish and leave it alone for 30-48 hours. Alternatively, some choose to soak their starfish in formalin, one part formaldehyde and five parts water. If you do this, be warned that the starfish will maintain a fairly strong chemical odor for a while until it dissipates somewhat. It might not matter if you're going to put it behind glass, but be aware. The process for this method is the same. Whatever series of steps you do to prepare the starfish and whatever you soak it in, you need to let it dry very thoroughly in the sun before taking it inside. Especially hot sunny days are perfect for drying starfish and ensuring that they'll last. Weigh them down with plates (you shouldn't need to weight them with books or any extra weight) to ensure that the arms stay flat and even. Check on them regularly to make sure it's drying in the shape you want, if you want it to look a certain way for display purposes. One easy alternative method of preserving the starfish is to lay it out flat on a plate and cover it with a generous sprinkling of natural sea salt. Top it with a plate to keep the arms straight. The salt crusting can work to suck any moisture out of the starfish and dry it out, preserving it quite nicely. You might consider doing this outside in the sun to avoid any smell and to dry it out more quickly.
Make sure the starfish you find are dead already. Clean your starfish. Preserve your starfish with alcohol. Dry the starfish in the sun. Try preserving starfish with salt.