Well, first it helps to have some vacation days (or no job at all). Plenty of time in your day to go looking for things. It's okay; if you get doubles you can sell them.
Most things require touring the worlds looking for spawn points. You will need to look in ALL of them to find everything. You will even need to go into space for some items and that means building a rocket. Others you will need to go on Vacation - Salvadorada requires multiple vacations. Frogs and fish can be used as pets. Obviously they're not real pets but they look like pets.
I also advise putting together a big collection room in your basement - I mean, you don't need to keep them but it sure gives you a good sense of self-satisfaction. It doesn't have to be very detailed; all it needs is lots of shelves. LOTS of shelves. There are some collections that can be stored specially, like the Elements and even a couple that are dedicated to Events that you may need to use BBitems if the event doesn't come back. When it comes to flowers you may want to use them to learn gardening rather than putting them on shelves.