Check the battery level on the recorder and make sure the level is high. If you are using a cassette recorder with no level indicator, press the record button and check the brightness of the red "recording" light. The light should glow brightly. If the light looks like it is fading, change the batteries. Most recorders use double "A" or triple "A" batteries.
Find out where the speaker will be sitting and try to get as close as possible to that spot to pick up the loudest signal for your recorder. Place your recorder on the edge of your desk or even on the lecturer's podium if that is allowed.
Turn on the "voice activated recording" button if your recorder is so equipped. That will turn the recorder on and off automatically according to whether the lecturer is talking or not. Otherwise, there should be a pause button located on the recorder that you can turn on to stop the recording and save batteries during long pauses.
Check the recorder periodically while the lecturer is talking to make sure the "recording" indicator is on. If you have a chance, you may want to listen to a portion of the tape to make sure it is recording properly.