Just keep flowing now matter what when your freestyling because it will enable you to rhyme off words you would have never thought of if you just gave up after you seem to not be to think up anything else.
Another good tip is to pretend like that you are not rhyming in sentences, that way you can rhyme off any word(s) in your flow not matter how long it takes you to find something to rhyme next with the previous flow.
Rapping smoothly and calmly but still loud and strong makes you sound like a better freestyler too. When you are freestyling you should rely on the energy of your audience if you have one or your own tight sick flows or punch line in order to keep yourself and your competition or audience hyped up. More than likely though your massive unlimited energy from you flow and punch lines will make your competition seem to be less intimidating and they probably will feel more intimidation from you.
You can always use DJ phrases like “pause,” “take that back,” rewind,” “take two or three,” and etc. in order to buy some more time, so you can think up some more killer phrases, punch lines, or rhymes to complement your rhymes and continue flowing smoothly.
Just keeping practicing over different beats (even ones that are not in the rap/Hip-Hop genre) in order to become a better freestyler. Eventually you will get comfortable with it and it will seem like killer punch lines and rhymes just come to you without you having to put much thought process input into it.
Before you know it you will be able to compete with some known rappers and still keep your cool without looking like an amateur in the game.