1. Doing Nothing
This is an unstructured practice of continuously "letting go", as I describe in The Easy Way - Do Nothing. The practice is to just be, letting go of whatever, if anything, is coming up, and if you can't let go of something, let that be okay as well. And if you lose mindfulness and come back, just go right back into letting go. The practice is to let go, and continue letting go. In essence to just stop. Stop doing and just be. Just this, nothing to do but just be, and always come back to just being. No need to grasp at anything or seek something better than this moment, just let everything be as it is. During formal meditation practice you've set things up so that for a period of time you have nothing to do. So do that - just be. Relax.
This has always been at the core of any good meditation practice, but in the past I thought of it as secondary to the need to maintain continuous awareness and I taught the letting go part after some awareness was established, but I'm beginning to think maybe people need to hear this first, so that this is always in the background from the beginning.
I would consider this to be an advanced style in the sense that many people will be spacing out left and right with this technique, and many may not get to what I would consider a full letting go, but I think this is nonetheless a worthwhile experiment - to begin to get it into the core of one's being that this is a valuable direction to lean in.
I wouldn't endlessly practice this as a standalone unless you are maintaining continuous mindfulness. Typically one or more of the following structured practices would be good to work on with an eye towards mastering the practice, i.e. putting in enough high quality practice until it becomes second nature.
2. Breath Awareness
The standard practice of following the breath. The main recommendation, the trick if you will, is to follow the sensate experiences of the breath, feeling the breath as opposed to thinking about the breath. The breath is always present and is used as a kind of anchor - when one loses awareness and returns to mindfulness, then return to simply experiencing the breath. Within the mindfulness of breath awareness practice, continue to practice letting go, just being.
3. Breath Counting
Same as breath awareness plus now we introduce counting the out-breaths. Continue to feel the breath, but now we count the out-breaths up to 10, and then start over at 1. The breath counting gives us a little extra to do, using up a little bit of mental bandwidth that we might otherwise use to wander. Counting can become relatively automatic, so ratcheting the count back to 1 gives us a way to avoid that automaticity, and provides some feedback if we notice the count is over 10 (or if we lose the count). Again, within the mindfulness of breath counting practice, continue to practice letting go, just being.
4. Breath Counting with Mantra
Same as breath counting plus now we introduce a mantra on the in-breath. A mantra can be any word or phrase, but I'm going to suggest "awareness" as a possible mantra. Although any word could be used, I recommend targeted words that are designed for whatever you feel you need for that particular meditation session. Words like maybe "peace", "love", "stillness", "relaxation", etc. However I do recommend that you pick a mantra for a particular session and stick with it. In other words, don't give yourself the wiggle room to change the mantra in that session as that just tends to add extra thoughts and doubts. Again, within the mindfulness of breath counting with mantra practice, continue to practice letting go, just being.
As you experiment with these 4 simple techniques, notice whether or not they help to keep you mindful. What percentage of the time are you actually mindful, aware of your awareness in meditation? (And within that, are you continuously relaxing, letting go?)
Even with a relatively high degree of this kind of structure, there is plenty of room to space out. A reasonably fit, relaxed person completes a breath about once every six seconds. Even with a mantra on the in-breath and counting on the out-breath, there are a couple of seconds to spare on each movement of the breath, which is an eternity in terms of the mind's inclination to immediately go off and wander. Sometimes you can notice that desire to go off and think. But the point is to commit to these continuous practices, and continually interrupt your mind's tendency to space out, and keep giving the mind experiences of continuous relaxed mindfulness.
Spending some time mastering one or more of the three structured practices might be a good way to develop a strong base before trying the noting practice I recommend in the series How To Meditate.