Adding to Karen's great advice, let me add that trying to tuck the first time you really want to be tucked is not the best of ideas. When we suggest "practice" we mean keep trying, but we also mean that when the tuck fails, try to note why it failed, how fast it failed, and if it was an activity that rearranged the package, was it the time of day, was it the particular pantie, were you larger or smaller when you tucked, or any number of variables. Knowing how it went wrong is as important as smiling when it went right!
I'm also about ready to have a burst of Tina-time, and it's been a while, so I've been going through my pantie drawer and seeing what still works and what doesn't. I used to stay tucked with just panties all the time, and I'm finding that after not doing it for a while, I'm not staying tucked all the time. But, it's getting better fast, and some of my older panties have gotten a little looser and just don't work as well anymore. You need enough tight material to cover everything (if you aren't taping) so the type of pantie counts.
As I type this, I'm wearing a standard low-rise bikini and have been tucked for 4 hours, and the tight jeans still look great :)
Most of all, have fun with it and it will get easier.
tina