jamesc's answer would often be the most convenient, but for when you know ahead of time that you'll want to use autocomplete for a command or search string, or other functions only available in a buffer, here's another way to get to the appropriate buffers:
- Type q : to open the command buffer
- Type a to start entering the new command:
- Use Ctrl+N/Ctrl+P for keyword completion, just as in jamesc's answer
Basically, q : does the same thing as : Ctrl+F: let you enter commands as if you're entering text in a buffer. Similarly, q / and / Ctrl+F both let you type search strings in a buffer.