How do I remount a filesystem as read/write?

The correct syntax is:

sudo mount -o remount,rw /partition/identifier /mount/point

Where mount/point is /partition/identifier's corresponding mountpoint, as listed by the following command:

mount -v | grep "^/" | awk '{print "\nPartition identifier: " $1  "\n Mountpoint: "  $3}'

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For example, say that the above command gives this:

Partition identifier: /dev/sda1
 Mountpoint: /    

Partition identifier: /dev/sda2
 Mountpoint: /boot

Partition identifier: /dev/sda3
 Mountpoint: /test

The following would be the correct syntax. (We start by unmounting it, if it's already mounted.)

sudo umount /test
sudo umount /dev/sdb3
sudo mount -t hfsplus -o rw,remount -force /dev/sdb3 /media/untitled

for busybox/android users:

Oddly, I needed to add a space (in contrast to normal usage) between 'remount' and 'rw':

mount -o remount, rw /

otherwise it wouldn't work.

UPDATE: it seems that this is almost never the case (see comments). Not sure which busybox-version + android-version I was running. I'll just leave this here in case anyone still runs into it.

Running dmesg | grep hfs showed that the filesystem was unmounted incorrectly, which I was able to repair using

fsck.hfsplus /dev/sdb3/