I am a long time DP/DR suffer. [...], there are two tricks I've learned that help (there are others but these are a quick two).
1) Touch two different surfaces at once, making sure they are as different as possible (e.g. cloth vs. wood). This forces your mind to process that and focus on the now, on reality. It grounds you.
2) Practice intentionally ignoring things. My particular method of practice was to take a coke can, set it next to my monitor where I could see it, yet refuse to look at it. Whenever your thoughts go to the coke can distract them by focusing on something else. Eventually I learned to intentionally ignore my DP/DR until it settled down.
I am very new here, and still reading and getting up to speed on the rules. This does not in anyway qualify as medical advice simply my personal experiences in dealing with what I know to be a very difficult issue.