A better and more sustainable method than the TX accelerator would be CPFP ("child pays for parent"). It's a little bit more difficult to understand, but I'll try my best:
If you receive a transaction which won't confirm: Send the coins you received (the easiest method to make sure you "catch" them, you can send the entire balance of the wallet - minus fee - or only a little bit less, if you're more advanced you can use "coin control" and "raw transactions") to yourself, but with a higher fee. The transaction with the higher fee will "pull" the other transaction into "confirmed" land.
If you are sending a transaction and your fee was too small, you can do the same if you get "change" from it. You must try to send the change (again, the easiest way is to send the entire balance of the wallet minus fee) with a higher fee to yourself, and so your sent transaction benefits from it, as technically it's the same method than above.
With Bitcoin Core that's all a bit tricky, as it doesn't like it to spend unconfirmed coins, read the details here:
https://www.reddit.com/r/Bitcoin/comments/4odi90/can_someone_explain_cpfp_child_pays_for_parent/Edit: A very good post is also
this thread which explains even more methods.
