Is hiring a proofreader beneficial in the long term?
What improves most your English level (or help you maintain it if you have reached a certain level which you deem satisfactory) is usage of the language in all its forms: listening, speaking, reading, writing. This is especially easy in this age of globalization and technology: just keep reading texts of all kinds, listening to the radio, watching TV programs in original language (BBC news, series, …), professional podcasts or videocasts, etc.

In addition, comparing the proofread text of articles with your original version can help you better understand some of the mistakes you make, the invalid constructions you may use and 
the shibboleths that give you away as a non-native speaker. However, unless you write (and get proofread) a large number of papers, this might not be enough to improve significantly.

Another possibility is to pay a teacher to actually review with you, on a regular basis, texts that you have written. Although it takes quite a bit of time, I believe it is one of the best ways to improve your written English, along with reading a lot.