Add some more RAM into your computer. To figure out the type of RAM you need, consult the manual for your motherboard. It should tell you exactly what kind of RAM is compatible and how many slots are free. Digital Audio Workstation software will take up a lot of memory. The more RAM that you have, the smoother your Digital Audio Workstation will run.
Render MIDI tracks with a lot of VST effects in your Digital Audio Workstation to WAV.Tracks with a lot of effects will slow your computer down significantly. By rendering it to a WAV file, you will be putting less of a load on your computer. Some Digital Audio Workstations have an option allowing you to "freeze" a track. This means that it will automatically render it to WAV for you without using any external software. Right-click the desired track that you want to freeze and, if your Digital Audio Workstation supports freezing, select the "Freeze" option.
Purchase a set of stereo monitors to mix and master your music. Stereo monitors have a flat response, so your mixes will be more accurate. If you do not currently use stereo monitors, then chances are your audio won't be accurate and will sound different in other sound sources. It is essential to mix and master your tracks through a set of accurate studio monitors. KRK and M-Audio both are reputable brands that make good studio monitors.
Close all processes that you are not using before opening your Digital Audio Workstation. Digital Audio Workstations require immense amounts of CPU and RAM, so be sure to close any unused programs that are using them up, too.