Could you tell us a bit about your line, stats, attenuation, type of DSL, line length and your ISP?
There might be things that can be done, depending on how much cash you want to waste, with no guarantees of improvement in some cases. I have three extremely long lines 4.55mi 7300m long, 65dB downstream attn, ADSL2 and have put a great deal of effort into making a load of changes, very speculative, some voodoo, some not, trying everything I possibly can to get more speed and reliability. So I can tell you what I have done.