The method Brad suggested is how I do it. Make two copies of the object and measure to the half way point on each. Make a box, larger than your part, and align it to the half way measurement point. Half of your model should be inside the box at this point.
Select all of that, right click and choose Intersect Faces (with model.) Then select and delete the boxed half. Repeat on the opposite side of the copy you made earlier. Make sure to save before starting this process as SU is finicky. Also, if the part is the same on both sides, then you can just do the one half and flip it. Now you can add pegs & holes, dove tails, clips, etc to join the two halves after printing. Sometimes SU is weird and deletes some of the faces where you split it in half and you will need to redraw the perimeter of the face.
To make a negative for casting in foam or silicone (which you can use for metal casting,) it is almost the same process. This time just make a box larger than the part (I usually go with half or quarter inch margins,) measure the half way point on the part and align this point to the face of the box you made. Again, the part should be 50/50 in the box and out of it. Select all of this and Intersect the faces. Now you can delete everything above the face of the box and the top faces on the box. You should be left with a box and the negative half of your part. Repeat this process for the other half or flip/mirror if appropriate. Once you have both halves then you can add keys to the boxes so they line up properly, just like regular mould making.
Hope this helps!