Most airlines will arrange for unaccompanied luggage service which is sent on a 'space available' basis. I think however that you need to have a ticket with that airline on the same route as you are sending your luggage...(at least that was my experience with Qantas)...the luggage MIGHT travel on the same flight as you, it could travel a few days later...
Problem I had though was that when unaccompanied luggage arrived for me in London, it wasn't shipped to Dublin for ages. It apparently went missing in a British Midland depot...However I was called to Dublin airport several times by the receiving agent here who said it had arrived only to go out there and discover it had not arrived (it was just listed as a freight item on some flights)The handling agent behaved disgustingly, treating me like an idiot when they couldn't find the luggage (even tho someone in their office kept phoning me to say it had arrived!) ...Eventually I had a phone call from a really nice guy, deep in the dark depths of some British Midland depot in LHR saying he'd found my luggage with no 'official barcode' on it...what should he do with it..I asked him to send it as freight to the handling agents offices in Dublin. He said that he didn't know how to deal with "freight" only luggage...So he said he'd sent it later that week on a passenger flight...three hours later I got a call from baggage services in Dublin to say it had arrived....Makes you wonder if the airlines really do know what goes as freight/luggage and if it is really accounted for?