The Nightmare Next Door

Their weeds are tall enough to get on the scary rides at the carnival. Their house is a retina-roasting goldenrod colour. They seem to throw loud parties on every day that ends in Y, and you're grateful if the worst things you find in your bushes are empty beer cans. Their renovation has been going on so long that you're beginning to forget you ever had a view of anything besides their dumpster. Maybe they just have so much junk strewn around outside that you can't tell if they're having a perpetual yard sale or evacuating.
Nightmare neighbours: if you're trying to move away from them, it can be difficult to convince anyone else to live near them. How are you supposed to sell your house when you've obviously got problem neighbours? No matter how delightful your home is, no matter how well it shows inside, potential buyers are probably going to notice the three rusty boat-tailed Buicks on the lawn two doors down.
The first course of action should always be to try to discuss the problem tactfully with your neighbours. Approach them with the benefit of the doubt: they probably don't realize how their behaviour is affecting you. Don't assume they are horrible, selfish people. Avoid speaking to them in a state of anger, if at all possible; this will only escalate any conflict. And don't make threats.
Explain that you are trying to sell your home and ask if you can discuss ways to make the sale go more smoothly. Be tactful, and have a few reasonable solutions ready for the problem(s) at hand. For instance, if their yard has been sorely neglected, and they're unable to do anything about it because they're dealing with serious illness or other problems, you might offer to pay for a lawn service to shape things up. No, it's not your responsibility, but it's a very small investment in getting your house sold.
If your neighbours are violating a bylaw, and you can't find a resolution by dealing with them directly, you can involve bylaw officers. They will investigate complaints and try to achieve voluntary compliance with local bylaws. Failing that, they have the power to issue tickets, order impoundments, and to initiate court proceedings. This can take some time, however, and if you're trying to sell your house, results may not occur quickly enough. If you can foresee selling your home, and think you'll need bylaw enforcement, contact officials as early as possible.
If the problem doesn't come under the purview of bylaws—for instance, it's a matter of aesthetics (the bungalow painted goldenrod, or the Christmas decorations that are merrily twinkling in August), you may not have much recourse. If your neighbourhood, development or building is governed by a homeowner's association, ask for their assistance. If you can't get cooperation from your neighbours on aesthetic issues, focus on protecting your view through the creative use of privacy fences, landscaping and window treatments.
Finding it impossible to come to a resolution with your neighbours? Consider mediation before pursuing legal action. A skilled mediator can help disputing parties find common ground and workable solutions they may not have discovered on their own. Mediators don't make decisions about who's right or wrong, but can help parties come to an agreement. And mediation is typically far more affordable than legal action. Many communities offer mediation services on a sliding scale or even free, through volunteer mediator programs, and they usually handle lots of "problem neighbour" cases.
As a last resort, you may have to call the police or take legal action. A lawsuit will typically take a fair bit of time and money, and police involvement usually sours relations permanently. These are desperate measures, but such action may allow you to get your house sold.
Finally, don't forget to spread good neighbour karma yourself.
Make the effort to welcome new neighbours with a friendly note or visit, and perhaps a modest housewarming gift such as a small houseplant or fresh baked goods. Such gestures can go a long way to establishing warm relations and smoothing over future difficulties.
Maintain your property's exterior. You don't have to hire a gardener, but keep the lawn trimmed, fix the shutter that's hanging at a 45-degree angle, and repair the roof before the shingles start hurling themselves to the ground. No matter how much you look at the outside of your house, your neighbours have to look at it more. Don't let it be an eyesore.
Be courteous: whether you are planning a big party, building a fence, removing a tree, or commencing major renovations, it's basic courtesy to inform your neighbours about the situation. This way, they can make plans to avoid the situation or offer input on how you can minimize the effect on them. Take responsibility for your actions: if building your deck means their car is covered in sawdust for weeks, you might give them some car wash coupons. If your new Doberman is terrorizing their tots, maybe you need to build a dog run.
Communication and courtesy are the keys to positive relations with your neighbours.