How to do we get khushoo in our salah? Sometimes I feel like I'm not completely focused and I start thinking of things that aren't important.
I have found that my feeling of closeness to God outside the prayer is what matters most when it comes to whether I can focus on the prayer or not. And that closeness comes from performing an extra amount of worship every day. Staying close to God requires daily work. If last night I spent an hour reading or listening to the Quran, then today I can focus much better on the prayer and enjoy it more.
For more on performing extra worship see my essay: Mysticism without Sufism: A Guide to Tahajjud, Islam’s Meditation Practice