For better upstream speed, ask your ISP if they offer the 832Kbit speed on any of their packages though often there is an extra cost for this (or there used to be, it is a while since I upgraded beyond 8Mbit so things may have changed) as it was usually a feature of more business oriented connections.
Line bonding is an option, but an expensive and not so efficient one, so even if there is a bump in price of 832K up you'll find that preferable financially if upstream speed is your main issue.
If you are Linux literate you could try putting a small box (one of those little low-power "plug PC" units would do fine, or some routers with custom firmware will do the job too) then you could try something like
this. That is an old reference, but still relevant.
This is a slightly more recent example. The key is limiting the amount of upstream bandwidth larger and/or bulk packets are permitted to use (fair apportioning of that allocation is a secondary task and may be a complexity you don't need) so that smaller packets (the packet acknowledgements and other such, delays to which are what cause everything to get throttled when one or both directions are saturated) to slip through quicker - as such pre-made scripts like
this may be all you need. Wondershaper is a common option in this regard and is included in the Debian and Ubuntu repositories:
http://packages.ubuntu.com/natty/wondershaper
Of course you could just get a private circuit installed, but as you said elsewhere that you don't want to bond two lines because of cost I'm guessing ~9K/yr for a 10Mbit (both ways) link is out of the question!