Mentally rehearse, visuallize yourself making those lifts, imagine how it will feel and remember your key points of form.
Think positive and psyche yourself up all day.
Besides that, as mentioned above, eat light before, you want to go to the gym on a near empty stomach, and stay hydrated. I like to have some caffiene before I lift.
Something I have been doing lately is rubbing Icy Hot (or an equivalant) on my knees beforehand. This takes away the dull ache I feel, and warms up the joints. It could be all mental, but it helps me.