A trick or a tip may work for some people, but I recommend you simply learn the mechanics.
Rolling the R is not a mysterious talent--it's simply a skill you can learn. You just need to have it broken down into simple steps that you can follow.
When we teach the rolled R, we start with three lessons, and 7 exercises:
LESSON 1. Develop tongue awareness
Don't skip this step. Most or all the difficulty comes down to lack of awareness of what's going on inside the mouth.
Exercise 1: The peanut-butter scrape
Imagine you have peanut butter on the roof of your mouth. Scrape it off with the tip of your tongue, exploring the entire inside of your mouth.
Exercise 2: The alphabet
Say each letter of the alphabet and take the time to feel exactly where your tongue is placed and/or what motion it follows.
LESSON 2. Learn to vibrate the lips and tongue
Don't tackle the alveolar trill until you really understand the mechanics of the easier trills. Every trill has these three things:
- Air flow
- Air gap
- Vibrating body part
Exercise 3: The lip trill
The easiest trill to start with is the lip trill. This is the sound we make when we go "Brrr--it's cold". Blow air out between your lips while gradually closing them, holding them in a relaxed state.
Exercise 4: The closed tongue trill
With your mouth closed, create a small gap between the blade of your tongue and the roof of your mouth. Blow out and you will hear something like a "Shhhh" sound. Keeping your tongue relaxed, close the gap until it is almost shut. Your tongue will begin to vibrate.
Exercise 5: The alveolar trill
While doing the closed trill, gradually open your mouth, keeping just the tip of your tongue tied to the alveolar ridge. Use the same technique to make the tip of your tongue vibrate.
LESSON 3. Incorporate the trill into actual words
Once you can make a pure, alveolar trill, you'll need to practice incorporating it into words.
Exercise 6: Vowel + Trill
Practice alternating between an "Ahhh" sound and your alveolar trill. Finally, connect them.
Exercise 7: Consonant + Vowel + Trill
Now it's a simple step to add words:
dar (to give)
Of course, there's still lots more that you can do to refine your sound and make it more native-like.
But, as others have pointed out, the rolled R is acquired later by native speakers than any other sound. And it's quite common for native speakers to need help learning it. So don't expect to master it overnight!
But with a modest amount of systematic practice, anyone can learn it.