There is a psychological experiment, recently reproduced for a television programme, in which some human guinea pigs are asked to see how long they can maintain a squatting position with their backs against a wall - as if they are sitting on a chair, but with no chair to support them. (This is harder to do than you might think and very good training for the legs!) One group is asked to see how long they can sustain the squatting position without any distractions. Another group also maintains the posture, but in the company of a plate of freshly baked and warm cookies which waft their delicious odour around the room. On average, the group without the cookies manage longer squat times than the group who have an alternative and far more rewarding temptation that needs to be resisted.
The point of this experiment is to demonstrate that human willpower is a very limited resource and that even something as simple as the smell of cookies can divert our energies from sticking at a straightforward test of endurance. Willpower is therefore not in itself a very good basis on which to build our ability to stick at anything and that if we are using it as the main driving force to motivate ourselves to undertake anything, we are likely to fail. As tai chi students, therefore, it suggests that we need to find other motivators to help us sustain our desire to practise, and I would argue that this is particularly important in the early stages when it becomes obvious that the rewards of practising tend to come quite slowly at first and in small increments.
If you have a partner who also comes to class, practising together can help - we have a natural tendency not to want to let another person down or to be seen to be "wimping out". Routine also helps - try to find a regular time and place to run through some of the class material, even if these are short sessions split over time rather than one long, dedicated session. Making your practice a habit will greatly increase the chances of you maintaining it: for more on this see here. Consider your environment: I train on our patio in the winter, which is too small for my needs but does have a nice secluded feel and catches the sunlight (if there is any) at certain times of the day. If you need to fit your practice in during breaks at work, maybe think about putting up an attractive poster, perhaps a cheap print of a Chinese painting, to help you feel in the right mood.
But the most important thing is to find a way of taking pleasure in your practice: once it becomes a bore or a chore, it will not last - witness all those gym memberships taken out in the New Year which will already have fallen into abeyance because the tedious grind on noisy treadmill in the end simply doesn't appeal. Practise in a way that makes you feel good, work at what appeals to you on the day and don't worry too much about "getting it right" - this will cause you to relax mentally and that will show itself in the physical quality of your movement because the mind and the body are really one.
Andrew Howard is a Chen-style Tai Chi Instructor teaching in West Dorset, UK.