While the procedure is basically the same between freshwater and water with salt in it there are a few added steps needed to ensure that the fish acclimates safely.

Saltwater is thicker than freshwater and being thicker it can have varying degrees of thickness this thickness is measured by the water's specific gravity or SG from 1.000 to 1.030 (1.000 being freshwater and 1.030 being very heavy marine). This is why the acclimation process is a bit more complicated when your tank water has a measurable amount of salt added to it. The salt makes the water harder for the fish to breathe so if the SG in your tank is a few points higher than the water the new fish is used too it can greatly stress the fish and even cause it to suffocate. For this reason fish being introduced to water with measurable SG should be introduced with extreme caution.

When preparing to add a new brackish or marine fish it is important to have your SG meter close at hand because both the SG in your tank and the SG of the water the new fish is in needs to be tested. If the SG of both your tank and the new fish's water are with in 0.002 of each other then following the same procedure for freshwater acclimation is fine except add water to the bag/container every 15-30 minuets instead of ever 5-10minuets.

Recently many stores have begun selling brackish water fish as freshwater fish and marine fish in extremely low SG of 1.014 instead of recommended 1.025. This causes many problems for the hobbyist when they get their fish home as a fast acclimation can lead too dead fish. If you call and ask most LFS they will tell you what they are running their SG at which will help you prepare for the difference in SG. Most fish will handle increases in salinity if done correctly some can handle drastic jumps in SG while others need very slow acclimation. If you don't know how your particular fish will handle drastic changes in SG it is best to do it as slow as possible. There are three basic ways a person can raise their SG safely and slow enough not to harm the fish quarantine tank (QT), slow drip, and long slow bag acclimation.

QT acclimation
The best and probably the safest way to acclimate new fish is over a period of weeks in a quarantine tank. Not only does this allow the fish to be separated so you can check for diseases but it also give a perfect opportunity to slowly raise/lower the SG over a period of weeks which is the most recommended way of doing it. Before you purchase your fish setup the QT and get it running at your normal SG of your other tanks. Then when you purchase your fish measure the SG of the water your fish is currently in if the difference is substantial lower/raise the SG of the QT to match the water in the bag. Then acclimate the fish to the QT using the freshwater acclimation procedure. Then over the next few weeks do 10% water changes no more than every other day and make sure the water you are adding matches the SG of the tank you eventually want to add the fish too. Once the SG matches the tank you want to add the new fish to your ready to re-bag the fish and acclimate him to the new tank. This is also the way to raise the SG of a freshwater tank to be brackish.

Slow Drip
What if you don't have a QT? Well then the next best method is the slow drip. This is done, by placing the fish in a container such as a pitcher or bucket and using a drip bag slowly drip water into the container raising/lowering the SG over several hours. The drip bag is just an IV type bag that opens at the top so you can add water. These bags are sold at LFS for adding Kalkswater to reef tanks but can be purchased from medical supply shops for about $2 they are called tube feeding bags or Kangaroo bags. They should come with a drip control valve, which is essential to control the amount of water being added per minuet. Once you setup the container and bag add tank water from the tank you want the fish to go into and set the drip valve to about 10 drips per minuet. If your container becomes full empty about 3/4 out and keep letting it drip till the SG matches the new tank.

Slow Bag
If your don't have a QT or drip bag you can do it right in the transfer bag too. This is not recommended as most people get impatient and add too much water too quick and end up over doing it. The key here is time raising the SG from 1.014 to 1.025 can take upwards of 6-12 hours depending on how the fish reacts. The procedure is the same as basic freshwater acclimation only instead of adding a 1/2 cup every 5-10 minuets you add 1/4 cup every 30 minuets. The important thing is to watch the fish if they start freaking out in the bag and breathing heavy add a little fresh water to bring the SG back down a hair and let the fish rest for a while.

Conclusion

Acclimation when done correctly can greatly reduce stress and stress related diseases in new fish. Acclimation while time consuming and tedious is the key to health and happy new fish.