(I posted a really good reply then it lost it, grrr.)
Like rara avis, I have schizophrenia and hear voices. I agree finding solutions/strategies is the best thing to do other than taking medication (if you are on any, I must admit I dont know much about BPD).
Challenging them with logic does not work for me, or anyone else challenging them, as it makes them more and more angrier, which in turn gets me more and more angry/frustrated. For example when the voices were really bad a couple of months ago and my CPN was challenging them, they were telling me (really strongly) to hit her. (I didnt, thankfully!)
Listening to music and other coping strategies. My ex-social worker gave a printed off copy of this link below, and told me to circle the strategies which were appropriate. Some arent relevant (especially as this is an Aussie link!) but some will be. If you find your own which are not listed, you could write on the back. For example this year I have found that baking cakes/biscuits is a good one (I give them away so I dont eat them all!). Other things I have found (again not listed!) is that getting your mobile phone out (especially good if you are stuck in a queue and are being bothered) and "pretend" to have a conversation with someone on your phone. Only you are talking to your voices, not someone else. (Only make sure you have it on silent or something, because otherwise it may happen what happened to me once!! I was in the post office, only my phone went off whilst I was having a conversation with the "voices", and because it wasnt on silent the ring tone went!! The whole of the Post Office queue looked at me like I was crazy and OMG I died of embarrassment. I can laugh about it now but it was highly embarrassing at the time) Another distraction I have found (and I dont care how babyish this sounds because it works for me) is doing in colouring books with coloured pencils, which is a decent task and been as you are unlikely to show anyone people dont care if you have a purple elephant for example. The point is that it is focussing your mind onto something else than the voices.
http://hvna.net.au/upfile/Coping_Stratigies_Poster.pdf
Listening to music is one of the best suggestions I have found. I use my ipod every single day and would be lost without it. You will find your own in time.
One thing I have found is focussing and repeating a sentence in Welsh (though any other language will do), but I only do that when I have time to sit around still for ages, because I find it takes time and energy. Because both the voices (I have two when things are bad) are speaking to me in English, they get confused, and eventually (after time and persistance) the second voice shuts up, after abusing me for a while. I have a "welsh" playlist on my ipod (it's full of songs which are in the Welsh language), and repeating and focussing on the song lyrics helps.