Soft wood = rotten wood (I cannot stop chuckling like an immature school boy!) Rotted wood will absorb water like a sponge and literally crumble away as soon as you pick it up. Here are a few steps you may have missed when you read the other posts. Here is a good summary of how to select and cure driftwood.
1. Take a walk and try to find a fallen tree that has been baked in the sun for some time. The wood will most likely have a gray hue to it, and look fairly smooth. Find dead hardwoods, e.g. Cherry, Hickory, Ash, Maple, and Oak. Maple and Oak will have more tannins as opposed to Cherry or Hickory, so I would only use them if I can't find an alternative. Oak and Ash are very dense and do not absorb water as quickly, so sinking may also pose a problem. Do not use Pine, Cedar, Fir, or Spruce. While these woods pose little threat if absolutely cured correctly, they can kill fish if introduced to a tank while still "green". Do not cut a piece off of a growing tree, or use a piece from a tree that has recently fallen. These pieces are "green" as well, and can be very hard to make into a piece of driftwood.
2. After finding your ideal piece of driftwood, check the piece for any soft spots. tap the wood with your finger and listen/feel for sound and density changes. Usually rotted areas are fairly evident, but a thorough examination is a good idea. Also observe any discolored areas. Discoloration can often mean contamination, i.e. oil, etc.
3. Use a putty knife (or other similar tool) to remove all the soft/rotted areas back to solid wood. Clean off all other debris like moss, dirt, vines, etc. Visually inspect the piece again to ensure you have removed any soft material.
Next, you need to cure the piece.
4. For a small piece of driftwood - Find a stock pot (or something similar) and fill it with water. DO NOT FILL THE POT COMPLETELY! Place the piece of wood in the pot, and begin to heat to a slow boil. Adding salt at this stage will also help to kill off any unwanted stuff. If the piece of wood must be weighed down, DO IT BEFORE YOU START HEATING THE WATER! Do not boil the water first, and then add the wood. Heat the piece for no less than 20-30 minutes. Most bacteria and other organisms will not live in boiling water for more than 5 minutes. You can heat the piece longer if you wish, but the point of boiling is to kill off harmful organisms and to help stop the decaying process. This happens in a relatively short period of time. After boiling, place the piece in a rubbermaid container filled with hot water and a high level of salt (read below). Let it soak until it sinks. Change the water from time to time to remove tannins (causing the water to turn brown)
For a large piece of driftwood - Find a trash can or rubbermaid container that will accomodate the piece. Place the piece inside and and weigh it down using rocks. Using the hottest water you can find, fill the container. Add salt. Cover. Continue to change the water daily with new hot water. Do this for one week. After that week, fill the container and add a high level of salt, say at least one cup per 30 gallons. Let the piece sit in the water until it begins to sink on its own. Change the water from time to time to remove tannins.
5. After soaking, remove the piece and give it one last general inspection. Things to look for - soft spots, oily residue, white grease-like spots. If everything looks ok, add it to your tank! (It may still try to float for a while, so may need to weigh it down.)
These are the things that I have done to cure large pieces of driftwood (36"X24"X18" and 65lbs) for my 300 gallon tank. It worked very well. My pieces are Cherry.