The easiest way I know, when you're at home or outside and don't have any gadgets or items, is simply placing your phone next to a wall or ideally in a corner with the speakers facing the wall. The sound waves will reflect off the wall and more of them will get directly into your ear.
To understand this, imagine a speaker in the middle of a room with you standing on the right side of the room. The sound waves move in all directions and only one part of them will directly get into your ear. The rest will first reflect off the left, top and bottom walls before they get into your ear. They will lose some of their loudness and will get there with some delay: You here a slight echo (of course depending on the size of the room).
So if the speaker is in the corner, more of the sound waves get directly into your ear or only with a minimal delay.
It isn't that much louder, than placing it in a bowl or toilet paper roll but you will still hear a difference.