We have been going to WDW for the past 12 years and they seem to rise at least once a year if not twice a few times. I don't remember them ever going down. But I can't say what might have happened before this time period.
The only "down" that has ever occurred is when Disney has done some special, short term discounts for tickets, e.g., like Fall 2012's sale of the PAP to DVC members for $399.
Be careful about buying a wad of tickets just because of price increases; especially if you plan on purchasing several years in advance. When evaluating whether to make an advance purchase you need to take into account the opportunity cost of tying up money in tickets vs. putting that money into an investment or using it in another way. Using the link chalee94 provided thanks, one day ticket prices have increased 4.85% over the past five years. During that same time, Annual Passes and Premium Annual Passes have gone up 5.40% and 4.03% respectively. (BTW, if anyone is interested I also calculated the 10 year and lifetime averages for the three ticket types.) If you can find an investment vehicle, such as mutual funds or stocks, that provide a return greater than the average price increase then it may be worth delaying buying too far in advance. Keep in mind that prices over the shorter term, one or two years, may not reflect the average of a longer period. For example, the price for Annual Passes jumped up 10.6% in 2012 and 6.1% this past summer. These were unusually high increases based on past years probably because prices were held down or didn't increase during to the economic downturn of 2008-2011.
In any case, I would hold off buying too far in advance right now. Ticket prices usually increase between May and August and Disney announces in advance that prices are going up. If you keep an eye on the Disboards you'll know with enough time to spare when to buy.
At the beginning of 2005, when Disney went to the Magic Your Way tickets, the prices for base, hopper and Plus tickets went down. However the hopping, WPF&M and No Expire became options instead of automatic.