Brown, wilted leaves are a good sign you're overwatering. Make sure your snake plant is in a pot that has drainage holes, and only water it when the soil dries...
You need to remove dirt and any pests that might be on the leaves. Plants can breathe and photosynthesize better when their leaves are free from dirt and dust.
They could not be fresh. They should be picked the day you're giving them to someone. Keep them in fresh water in a cool area to help them live longer.
As with most other houseplants, the likely cause of browning tips on a peace lily is over or under watering. Peace lilies tend not to need much water, you can...