When taken to the limit, even a non verbal safeword is not necessarily practical. Many subs in subspace forget they have a signal. I know I have...

Anything that makes a noise or the Dom/me can feel are ideal. You can stamp a foot, knock on a board with your fist in a certain pattern, even press a button that rings a bell. I read one case where a stack of CDs was left near the sub where he could knock them off if he wanted to get out of bondage. Another way may be to drop an object held in the hand - a ball, something that clanks or makes noise when it falls and so on. Though this is prone to false positives as you get so turned on you lose control of your muscles and drop it anyway.

It is important in any play to ensure that the Dom/me is aware of the sub at all times - os capable of gaguing their reactions to things enough to know when to stop or slow down. They need to keep a check on breathing, circulation, skin colour and other signs that you may be in pysiological distress. It also helps if they occasionally ask questions that can be answered with a nod or shake of the head - possibly when they think you may be in discomfort and want to stop. Questions like 'Are you Ok?' and 'Would you like to stop' or 'Do you like that or not?'

Safewords and safesigns are generally a tool useful for roleplaying where it is true that when a sub says 'no' they really mean 'yes' and this is where you mainly use them. In most cases, where you are playing you can often just say to the sub 'do you want to stop now?' or the sub can ask you to stop normally. They should also not be your only method of safety, it needs to be supported by other techniques such as keeping a check on the subs state of mind.