If you are willing to spend money, contact your ISP and tell them you want a faster connection. You will most likely have to get a better bandwidth/download-upload speed but that will drop you ping. If there is an optical cable leading to your house/flat, I'd suggest you use that. Of course, that requires you to get a contract with your ISP.
What you can do at home is, get the smallest TV/monitor possible. If you have a chance (but I don't think they still sell them, at least not in my country), get a Tube TV. They are much more responsive then the LED/LCD/plasma TV most of us use. Although if you have one of the latest LED TVs, with an 800 Hz refresh rate, I don't think its that much slower, but still, difference is noticeable. One of the options is to use the component/SCART cable that you got with the PS3, you know, the one that consists of 3 cables, white, red and yellow (not sure, haven't used it), which is supposed to be quicker than the HDMI. If you are running on Wi-Fi, ditch that too and get a cable to your PS3, be it via a powerline connector or laying a cable through the house... Be sure not to get the linksys powerline connector, I got it and it sucks (it's slow). Get the netgear 500mbps one. They range from 60 to 160$. Depends on the offer you can get.
Also, if you know your way with computers (just a little bit), asing a static IP to your PS3 and forward the ports GT5 uses. This is supposed to get the data directly to the PS3 without your router processing it beforehand. At least that is what I think it does...
Hope that helps you some
EDIT: Ping is the time that takes the data to be transfered from the server to your device. Ping highly depends on the location of the server. I usually get 16ms local ping from a server 20 miles away. If I do a test from a US server, NYC, it gets me 162-172ms ping. Test with a server in Sydney, 399-426ms ping. So this is the time it takes data to come from the server to me. I.e if an American ghost ghosts for me, the time that takes me to see the ghost is (ghosters ping, which doesn't matter because both draggers get the same delay due to it) + (my ping to the US server which is 172-226 depending on the server location).