General tiredness:
* As heavy as a lead weight.
* As worn out as an old shoe.
* As limp as a wet noodle.
* As drained as a battery.
* As sluggish as molasses.
* As slow as a snail.
Specific kinds of tired:
* Mentally tired: As empty as a balloon that's lost its air.
* Physically tired: As sore as a pulled muscle.
* Emotionally tired: As empty as a hollow shell.
* Sleepy tired: As heavy as a rock.
The best simile for you will depend on the specific context and the type of tiredness you want to convey.