For the query plan itself, you can force compilation every time using the following option on your query:
However I suspect what is happening is that the second and subsequent executions of the query are pulling the data from memory instead of disk, and of course memory is faster than disk.
If you are trying to test the scenario where this data always comes from disk, there's not really a good way to do that on your production instance without affecting anything else, because you can only drop clean buffers for the entire instance at a time, not for a single database, never mind table. So what you could do perhaps is set up an instance on the same hardware, with just this table, and run the following before every run of the query:
However, this does not make a lot of sense, as ideally you will be querying data that is in memory, and you should be optimizing for the best case scenario. You should have some idea of what the worst case scenario is like, but again, this should not be the normal situation, unless you have a very big database and a very small amount of memory (in which case, open the wallet and buy more memory - it's much cheaper than optimizing for the lack of it).