Even if you feel lazy on a weight day, do something. Decrease your routine or at least go to the spot where you lift and contemplate the activity. Simply showing up will help reinforce the habit. Stretch each muscle group before and particularly after your workout. This will help you avoid stiffness and maintain the flexibility and range of motion in your joints.
Moderation is the key.
Significant strength gains will happen even by working out with fairly light weights. You don't have to be trying to continually increase the amount of weight you lift. Do what is comfortable. The goal is simply to stay strong to insure a long, active life. Finally, moderate weight training is safe for most healthy seniors.