Herb it up
People are always telling you to eat more greens. Try starting with Calea zacatechichi, better known as the "dream herb." Used by the Chontal Indians of Mexico to induce mild hallucinogenic trips, the dream herb is one of the few (legal) herbs that might increase vivid-dream frequency. It's pretty easy to find in tea form, although you probably shouldn't have a tea party with it, unless you want that party to end up as a sleepover. Other herbs, like mugwort and Huperzine A, get a lot of hype in the lucid-dream world, but there's not much evidence to corroborate these claims.
If you're desperate, load up on meds
Your brain is a delicate balance of chemicals. So delicate, in fact, that even something as seemingly harmless as Nyquil can mess it up. While the exact cause is unclear, nightmares and wildly vivid dreams are a known side effect of Nyquil.
Nicotine patches, especially the 24-hour kind, have been shown to cause similar experiences. Nicotine has a unique relationship with brain activity and REM sleep, creating an environment ripe for not just having crazy dreams, but also for remembering those dreams in the morning. Most commonly prescribed drugs for mental and emotional disorders affect dreams in the same way. So if you're a flu-ridden recovering smoker on benzodiazepines, you're probably in for some wild rides.