I use slime made out of cake batter and water, but sometimes there is a long time between when I make it and when I use it. Has anyone found a good way to keep slime warm?

I've never kept it warm but, I've made it with hot water and made sure it was cool enough to use.

I put mine in a bucket and fill the bathtub with really hot water and let it sit in there absorbing the heat through the bucket. Stir it once in a while and drain the tub and refill with fresh hot water every 30 mins and in about 90 mins it'll be warm.

It depends on what you mean by a long time.

I'd say oilslick has the right idea to mix it much hotter than you want it to be when you use it. If it's still too cool by the time you use it, you'll need to insulate your slime container somehow.

The easiest way to do that would be either wrapping it up in a blanket or putting it inside a larger insulated container like a food delivery/catering bag or a cooler.

The hot water method that Pup Kaos describes would also work for this.

Good advice above. It also depends on how much. If you have some bowls or small containers...possibly put them in an oven on a very low / warm setting? I have a 6 gallon / 22L stainless steel pot for ...reasons... But it works great for warming, or keeping substances warm on the stove top. For more liquidy substances, I have an immersion heater which works pretty well. If you have a bucket or pail, you could also slow cooling by wrapping in a towel and placing another folded up towel on top to act as insulation.

The bathtub method is the best and easiest one for me.

Also, if your slime is sitting for a loooong time, it'll probably get thicker, so you can also boil some water and then add it to the slime to heat it up and "thin it out" in one fell swoop. Obviously you want to stir the hot water into the mixture... But give it 10 minutes to cool off first, or you WILL burn your hand.