The traditional approach to campaign finance limitations has been to go after the contributors, to limit who can contribute to a campaign and how much they can contribute. The rules are set to restrict certain types of organizations from donating too much or spending too much on behalf of a candidate and to specify limits on how much any individual can contribute to any candidate. This system is complex and inconsistent and full of loopholes. It permits bundling of donations, doesn't address certain types of contributions and allows for all sorts of in-kind and indirect spending which ought to be restricted. This system clearly doesn't work and every attempt to reform it makes it more complex and not significantly more effective.
The answer could be to approach it from the opposite direction. Instead of restricting who can contribute money and how much, why not just limit how much money a candidate can spend? Spending the millions of dollars some candidates spend just to get elected to Congress is excessive. Keep the limit appropriate to the significance of the office, but make it low enough so that incumbents and challengers can raise it without any great effort. Then ban all soft money and all spending for advertising by anyone other than the candidates campaigns. Make the candidates and their supporters work instead of just spending money. Get them out pressing the flesh and making speeches and earning their offices instead of going into debt or taking on more shady obligations.
Of course, this would cut back massively on advertising in the media for most campaigns, and that leads into the other problem that has to be addressed, media coverage. With advertising cut down, the coverage which the media voluntarily gives to candidates becomes enormously more important. The media could literally make or break campaigns, and despite their claims of impartiality, even something so small as the choice of which soundbyte to use or the amount of on-air time given to the incumbent over his challenger could create problems. This is the problem which has led to 'equal time' policies like the controversial and often proposed 'fairness doctrine', and they're a problem in and of themselves. They're as hard to apply fairly and evenly as the current campaign finance laws, and amount to a massive violation of press freedom, free trade and free speech.