Buying peerages or policies via donations to political parties is, in security terms, an "exploit". Unfortunately, it's not an easy one to fix, even aside from the fact that the people who would be attempting to fix the exploit are the same people who are benefitting from it, and thus have to reason to get the fix right - you wouldn't hire the guy who hacked your bank's software to write the fix to his own exploit (not without some serious review mechanisms).
Jock outlines a neat solution in this post. The reason it works is because it purports to vastly reduce the demand for cash by political parties by reducing all campaigns to face-to-face discussions. I'm not sure I totally buy this, incidentally - presuming that the party system were to stay intact in such a system (and I'm not at all sure that it would), I can see that parties could win more by spending money in a different way - instead of national advertising, training candidates on public speaking, debate, and so on, and providing high-quality campaigning materials such as powerpoint presentations (you just know that cellular democracy will result in campaigning-by-powerpoint), flyers, websites and so on. But anyway, I think it's a neat idea on other grounds anyway, so I share it with you. My three readers, one of whom is Jock anyway :P
I'd like to propose a different solution: eliminate the demand for donating to political parties. Many (not all) donors give large sums on the presumption if not outright agreement, that they will benefit somehow themselves. There are three obvious ways in which rich donors could benefit from donating either to the party in power, or the party who is about to come into power. Incidentally, this is why the Lib Dems have, I suspect, significantly fewer large donors - because they are unlikely in the short-to-medium term to gain power and be able to pay back favours.
1. A peerage or other honour: since the scandal, I suspect that the likelihood of rich donors being nominated for peerages will have dropped significantly. However, leave it a few years, and the parties will start to peddle them once more.
Solution: a sortitioned Lords. If the Lords is drawn at random from the public, there's no way to buy one's way in. For other honours, well, who cares? Knights don't get to rule anyone.
2. Favourable policy decisions: Lord Ashcroft seems to be bankrolling the Tories on the basis that they will bring in policies more to his liking.
Solution: a sortitioned Lords would help here, along with a written constitution and a transparent and well-managed lawmaking process that rejects spurious or bad laws. Giving MPs cash bonuses for sponsoring "good" laws would incentivize them to not just follow the party whip. Finally, a system of election that is more representative of the people's views would make parliamentary votes closer, meaning that governments would be less able to steamroller bad legislation through parliament, particularly if their own MPs stood to lose money on the proposition.
3. Favourable executive decisions: Local councillors seem to be being bribed all the time to make favourable property planning decisions.
Solution: separate the executive from the legislature. Ministers should not be responsible for policymaking, running their department, and representing their constituencies all at the same time. Instead, the heads of departments should be appointed by Commons committee and ratified by the Lords. An ombudsman with real power to investigate complaints and abuses in the departments, and to recall the head of department, could be appointed by the judiciary and ratified by a jury (because parliament should not be appointing both head and ombudsman - perhaps the Lords could appoint an ombudsman instead?).
Finally, the rules on donations at the moment need to go. They are just too easy to circumvent. There are only two sets of rules that I think would be easy to enforce and difficult to game: either no donations from 3rd parties at all, or no restrictions on donations at all. The former can still be gamed by gifts in kind (need somewhere to hold your conference? Use my hotel. Need someone to make your party political broadcast? Use my TV company!) so I'd tend towards the latter, presuming that my solutions were implemented.