Here are some things that I have used recently to assist in the cultivation of metta...
- Listening to chanting of the Metta Sutta - I prefer an English translation on "repeat" on my MP3 player. I ensure I fix my mind on what is being chanted. If I notice my attention wandered, sometimes I will rewind the sutta chant back to the start of the track and endeavour to concentrate better.
- Reading Mahasi Sayadaw's "Brahmavihara Dhamma"... whilst I'm not overly enthusiastic over his treatment of the 3 remaining brahma-vihara, his account on metta cultivation is good. (Likewise actually with the Visuddhimagga, now that I think about it). It's available free online @ Buddhanet.
- Bringing mindful attention to the intention/resolution that metta-bhavana is indeed an active component of your practice. If it's just a token gesture without sufficient commitment, there will be insufficient mindfulness (i.e. failure to develop a positive habit) to actually engage in the practice.
- Make it a Dhammic frame of reference more often. In the past I was more inclined to think and frame situations in terms of the 4NT, three characteristics, sankhara etc. I find that if this is not complemented with wholesome mindstates (which include metta) then it can actually be quite bleak and depressing. It's not much fun having moderate insight into the unsatisfactoriness of all dhammas if one does not have the means (incl. time, energy, wisdom) to attain stream-entry.
- Recognising that metta assists in calming and purifying the mind, so if/when you do engage in vipassana-oriented meditation, it serves as a useful prelude (either immediately before, or in the preceding days).
- Sometimes just smile and see if the mental catches up to the physical... interestingly it does.
They're the main things that come to mind at the moment. Note though that the common theme as that there is not strong emphasis on "sitting"... it is primarily about integration of metta into daily activities and interactions. It's a complement to the spiritual path and it's a complement to life.