Rinse your paint palette thoroughly under cold water to wipe up any excess paint that has not yet dried onto it. Do not use hot water, it will melt the plastic. Take your time as you rinse with the water to allow some of the paint to come off.
Use turpentine oil applied on a rag to remove fresh paint residue. Just put a slight dab of turpentine oil on the rag as you do not want to soak the entire thing and make a serious mess. When removing paint, take your time and apply a little pressure with the rag. You find that a lot of the paint will be removed from this technique. Always remember to use turpentine oil in a well ventilated area as the fumes are toxic.
For dried, stubborn paint stains, apply turpentine oil onto the affected area with the help of a clean rag. Let the turpentine saturate into the dried paint for almost half an hour. You will have to be a little patient to let the turpentine oil to do its job.
Once the dried oil paint has softened, use a razor blade to scrape the paint off the palette. For optimal scraping, keep the razor at an angle of 45 degrees. This will also prevent the palette from picking up any cut or scratches. Be very careful while handling a razor blade as you can get seriously cut from it. You may want to use a paint scraper that is specifically designed to remove dried up paint as it basically has a blade on it but it is attached to a convenient handle for safety and so that you can apply some decent pressure to it to help you.
After removing the paint from the palette, clean it again using a rag dipped in turpentine. Once you have cleaned up most of your palette it is a good idea to use the turpentine oil again with the rag to further remove any leftover paint.
To remove dried acrylic paint, use a fabric softener. Soak the palette in the fabric softener for several minutes. The fabric softener will soften the paint and it will come off quite easily. You can find fabric softener at any convenient store or you may have some laying around the house in the laundry room. Fabric softener is a great way to remove acrylic paint stains but sure not to use too much of it on your palette.
Alcohol also works well for acrylic paints. Soak the palette in alcohol for 15 to 20 minutes and then rinse off. You can basically use some rubbing alcohol as this works best to remove the acrylic paint. You might want to wear gloves while using the rubbing alcohol as it has a very strong odour.