Flushing the hot water heater is generally good preventative maintenance, but it won't fix your dip tube. Every hot water heater manufacturer recommends that it's customers flush their water heaters once a month just to remove dirt and scale from the heater. However, flushing your heater will not affect the hot water cooling down problem your first post was about.
The standard procedure for "flushing" a hot water heater is as simple as mud, and it goes as follows:
1. Simply open the drain valve of the water heater until the water runs clear.
2. Close the heater's drain valve.
3. Repeat Steps 1 and 2 once a month.
While Kok may be right that if you take the drain valve off, you can get into the heater with a wire and break up any lime or scale inside it, A. O. Smith makes an "Un-lime" kit to do that. Basically, it's a gallon jug of a mild acid that you connect the to the drain valve of your water heater.
The instructions for using the Un-lime kit can be found here:
Basically, the procedure is to:
A. Drain the water out of the hot water heater:
1. Turn the temperature control down to "Vacation"
2. Close the cold water inlet valve to your water heater.
3. Open a hot water faucet and the drain valve on the heater to allow the heater to drain
B. Use the Un-Lime kit:
4. Connect the hose from the Un-lime kit from the heater's drain valve to the gallon jug of mild acid.
5. Elevate the jug of acid so that it flows into the water heater, and leave it in there for 1 minute
6. Lower the jug of acid so that the acid flows out of the heater and back into the jug.
7. Repeat Steps 5 and 6 until the acid flowing back into the jug doesn't have any foam in it anymore. (That foam is CO2 gas, and it's the result of the acid dissolving calcium carbonate (which is lime scale) inside the water heater.)
C. Put the water heater back into operation:
8. Once there's no more foam in the acid, remove the Un-lime kit from the water heater, open the water supply valve to the water heater for a few seconds to flush out the spent acid. Then, close the drain valve and the hot water faucet. Open the water supply valve to the heater. Turn the thermostat back up, and bleed the air out of the hot water supply pipes.
D. Properly dispose of the spent acid
9. Wait until it gets dark, and go throw the spent jug of acid in your local river or lake.
The acid won't harm your drain valve or the tank. It only dissolves any lime scale you have inside the tank. You can contact A. O. Smith tech support in you want to know more about purchasing and using one of their Un-lime kits.
Removing the lime from your water heater will extend it's life because the lime will insulate the water from the source of the heat, and that causes the bottom of the heater to get hotter than it otherwise would. Removing the lime ensures that the bottom steel wall of the tank doesn't get any hotter than it should.
However, this is all fine and good, but you need to replace your dip tube to solve the problem.