The first is to assume your beliefs and causes will triumph. There may be solid evidence for this, as solid as Nevada's law banning gambling forever. (Passed 1909, took effect 1910.) Or the victory of monarchical power in Britain after Oliver Cromwell's death.
The second is to be certain your side will go down to defeat. The Satanic [liberals, conservatives, greens, pugoristas] will win. The Soviet Union will prevail over the Free World -- oops, that one doesn't work any more.
The third is to take for granted that future politics will be just like today's. This is plausible provided there are no new industries, geographic shifts of old industries, new technologies, population changes, major Supreme Court decisions, changes elsewhere in the world....
One way which looks easy: Assume the future will repeat the past. The US will fall apart the same way the Roman Empire did, and be succeeded by feudal societies. Knights will use flying cars rather than horses, and so forth.
To do this well (defined as "not making editors and readers laugh in the wrong places") requires a bit of historical study. It also requires thinking: what will be to the American Empire as Christianity was to the Roman Empire? What language will Canadian barbarians speak: French or Inuit? Will China, India, the EU, and Indonesia undergo the same transition?
What follows is based on these assumptions:
1. You want to avoid having your future become outdated before an editor reads your submission. (Setting your story centuries in the future won't help if you get the near future blazingly wrong. Think of the writers who took for granted that the Soviet Union be around for centuries, and had their submissions read after Russia seceded from the USSR.)
2. You like the idea of having your fiction reprinted all during your lifetime and earning you more money.
3. You want your fictional future to be different from everyone else's.
Begin by looking at what's already happened. The post-WW II US Baby Boom started in 1946. By 1950, it should have been obvious that around 1958 there would be a whole lot more high school students than came out of the Baby Bust. And a bit farther on, American colleges were going to be crowded. (The people in charge of planning for schools and colleges didn't see the obvious, by the way.)
In the 1950s, English writers wrote futures in which England was still a Great Power on Earth, and had become a Great Power in space. (Yes, I meant "England." It was then easy to forget there were other countries in the United Kingdom.) The UK was no longer a Great Power by then.
Investigate cycles. The United States has a conservative/liberal political cycle of about twenty or thirty years.
Then there are "moral panic" cycles. There are times of little worry about drunk driving, and times when it's considered a major problem. Periods when it's difficult to get police to care about accusations of pedophilia, and others when authorities believe 150% of such allegations.
And then there's the heroin/cocaine cycle. At one point, heroin will be the Big Bad Drug. There will be experts who say cocaine isn't really addictive, and doesn't cause nearly as much damage as heroin. Some heroin users will turn to the safer drug. At another point, cocaine will be the Evil Drug and heroin relatively harmless. (Cynics might find this similar to the conservative/liberal cycle.)
Investigate trends. The US reached its peak of relative economic power in 1945, and has undergone relative decline since then. This probably won't continue to the point where the US is the least prosperous nation, but it might continue for some decades.
And then you guess. What position in the political spectrum will go with which economic theory? Which religious affiliations are most likely to go with conservatism or liberalism? Who is most likely to be pleased, or most likely to be offended, by the results of Vatican III?
Oh -- then you do this again for the next story, giving it a new future.
Yes, there are advantages to keeping the same background; Robert A. Heinlein was able to use his for decades. But he had to postpone the Strike of 1956 to 1966, 1976, and then an indefinite time in the future, among other feats of fudging.