There's isn't really any trick to recruiting legends, other then being level 90+, having a friend bow, and some luck. There are some tricks to finding or getting to them, though.
Make sure you have the Music Box on hand if you're going after the regis and/or Mew. Check every single Pokemon on each floor, and also check large concentrations of items for monster houses, and keep an eye out for any "Mew used Transform" messages.
As for Jirachi, bring Frustation TMs, and use the floors with Skarmory (I believe 12-15 have them) to level up as much as you can before moving on. It's time consuming, but worthwhile.