One of my peeves is soggy, mushy bar soap. One, it's gross. Two, it makes a mess. Three, it's a waste of soap to let it melt away in a dish rather than being used. There are a few ways to prevent the mush and have a long-lasting bar of soap.
The biggest enemy of soap is moisture, so the key to a long lasting bar of soap is to keep it as dry as possible. All soap requires some liquid as an ingredient. The trick is to keep it to a minimum and allow it to cure properly. The longer a soap cures, the more moisture will evaporate and result in a harder bar. This is one reason I wrap my soaps in cloth: the cloth allows the soap to continue to harden even after it's wrapped.
One thing you can do at home is to allow your soap to harden is to store it away from the humid bathroom and, if it is packaged in plastic or other non-breathable material, unwrap it. You can take advantage of fragrant soaps by storing them in someplace like a linen closet or dresser drawer. That way, you'll scent your linens or clothes while hardening your soap.
Once you're ready to use your soap, consider where you put it. The absolute worst place is in the shower where the water will hit it continuously. Observe where the water flows and use a soap dish out of the water's path. If you don't mind an extra step, take it out of the shower when not in use. Personally, I don't do this or I would probably forget to grab it on my way in and have to step out dripping to get it.
Finally, the most important thing you can do to make your soap last is to let it dry out between uses. To accomplish this, you need a soap dish or surface that allows proper drainage. The best option is something that raises the soap up and allows water to drip away and air to circulate under the bar of soap. Something like this is good for a handmade option. If you already have a soap dish you like that doesn't drain well, I've found spiky plastic soap savers similar to this in packs of two at the dollar store. You can use them with a soap dish or alone on the counter. Depending on the shape of your soap, you can also rest the soap up on it's side rather than flat. This doesn't allow the soap to dry as well on that edge, but it does limit the surface area that stays damp. I've used all of the above methods to allow my soap to dry and have had success with each. I'm sure there are others I haven't tried.
Nobody likes to see money washed down the drain. Whether you buy your soap at a supermarket or handmade from a soap maker like me, I hope these tips help you to get the most out of your soap.