If you have to count beats in your head, you're already failing as a DJ. Knowing when to drop the next tune should come naturally.
Read some music theory if you have to, and definitely spend time listening more closely to your tunes. Try to think about how your music is structured as you're listening to it. Identify the intro, chorus, verses, bridge(s), etc.
With enough critical listening (and practice on the decks), you'll no longer have to count beats to know where you are in the song and when to start the mix. It'll eventually become second nature for you.