One suggestion is to use your laptop not directly placed on your lap, or even avoiding putting your laptop on a bed, couch or chair directly. It is a LAPtop after all, right? Why shouldn't we use it while curled up in our favorite chair or on the couch snuggled under a blanket or in bed?
The biggest reason to have something like a laptop desk, (a flat sometimes wood or plastic surface towards the bottom of the laptop), or a laptop rest that has either passive, (no fans), or active, (spinning fans), is to allow the air to freely circulate around the intake ports and the exhaust ports to allow air to cool the internal components of the laptop.
The two previous mentioned reasons for over-heating are correct and good reasons that laptops over-heat. We keep these portable devices we own tethered by always having the power supplies attached and charging the battery. The heat produced by constantly charging the battery when the battery is fully charged, or when the vents are blocked will also shorten the life of the battery and the laptop might become hot enough to damage other internal components. Dust is a big killer of laptops and, (almost more so), desktop computers.