There are a LOT of misconceptions about the flu shot, mostly based on innaccurate information and fear. I'm a strong proponent of the flu shot, because people are hospitalized and some die from Influenza every year.
The Mayo Clinic says that all women who are expecting to be pregnant during flu season SHOULD get the flu shot. With pregnancy your immune system is a bit compromised to begin with, and your heart and lungs can be taxed as well, so pregnant women are not only at a higher risk of catching the flu, but from suffering serious effects of the flu. Not to mention we PE moms have funky stuff going on in our bodies during pregnancy anyway.
Lots of people take lots of medications, and it's usually those who are the most ill that need the flu shot the most. This is why when there are shortages in the flu vaccine that children, the elderly, and those with chronic diseases and compromised immune systems get first dibs.
That being said, mkvh is correct that you shouldn't get the flu shot during your first trimester, but before pregnancy it is recommended and in your 2nd or 3rd trimester it is ok as well. And remember NOT to get the nasal spray vaccine as that uses a live virus, rather than an inactive or dead virus like the shot. Those who are allergic to eggs should not get the flu shot either.
It doesn't matter how long it's been since you've had the flu, as the flu virus strains change every year, so the flu shot changes every year, and you just never know when you might get it. ;-) It won't be effective against all the strains, just those predicted to be the most prevelant for the season as onesock notes above.
Please DO talk to your doctor about it. October and November are the best times to get the shot, though you can get it later and still receive benefits as peak flu season is often in January in the US. I was in the late 2nd trimester at the start of flu season, and my doctor was adamant that I get the flu shot. I had the shot several years in a row prior to pregnancy, and have had it since because I had a preemie at home, and it was again a requirement.
I've gotten flu-like illness 2 or 3 times, but it was a mild and abbreviated flu(or some other virus) that lasted only a couple of days.
Healthy adults should receive the vaccine when it's supply is readily available and not in shortage, because we can pass it on to the young, sick or immune comprimised, and the elderly, even if you don't get sick from it. Those who've gotten the flu shortly after receiving a shot were likely to have already had the flu virus when they got the shot, and it was a cruel coincidence....or they had another virus that mimics the flu symptoms, but was actually unrelated to the influenza virus. You can't actually get the flu from the flu shot, because it is a dead virus. Though you can experience mild fever and aches for a day or two. People die every year from the flu, and pregnant women are often hospitalized if they suffer serious effects of the flu...so I think it's worth it.
Here's some info from the Centers for Disease Control, too:
http://www.cdc.gov/flu/about/qa/thimerosal.htm (Thimerosal - part on pregnant women is several paragraphs down)
Good luck in your decision and your dicussion with your doctor!