I took ear training classes in college. I graduated with a BA in Music and IMO the ear training classes were most helpful along with voice lessons. I have a good relative pitch, but not so much perfect pitch.

Relative pitch-when you get the name of your starting note and can name other audible notes relative to that pitch.
Perfect pitch-play a blind note and name it.

Anyway, one of the best tips I ever had in developing interval recognition in relative pitch was to assign or remember familiar intervals to certain note intervals:

eg. "Here, Comes THe Bride" Perfect 4th (i.e. C to F) or beginning notes to Fur ELise, minor second. (E-Eb or D-C#)...

Our ear training teacher would give us the first note of a phrase and then play a melody. We would have to transcribe it. This is really a good exercise.

There are many others to use...anyone else want to offer some advice?

I remember such classes in high school where the teacher woudl sit at the piano facing us started playing and we have to write down what was played...

Always hated those, today I wish there was more tricks in that class in order to actually improve your earing of pitches.

Most of the time I was exempt of those theory classes because of my past lessons as a kid(did classical piano from 6-12, contests, shows etc)
So instead of going through boring theory classes that I already knew I got to blast on guitar in my booth

Some of my favorites ...

#4 (or b5) "Maria" - West side story
b7 - original "Star Trek" theme

major 3rd 'down' - "summertime"
perfect 5th 'down' - "feelings"