I assume your taking it the normal way (just taking it out of the container and swallowing them) the only thing you can really do is bypass the time release so the whole dose will take effect immediately rather than over the course of 12 hours, The easiest way to do this is:
1. Place the whole capsule under your tongue and dissolve/absorb the coating
2. Take it out and notice there's a black half and a white half
3. Cut it in half and then take the white half and cut it in half again
4. Swallow those 2 pieces and either throw out or swallow the black piece (doesn't really matter)
This will last maybe 4-5 hours..maybe 6 at the very max if your lucky.
If you really want to keep the extended release effect there's a few things you can do that may help
(note: these things should also help if you take it the way described above)
-Take it on an empty stomach*
(*it does help but I dont recommend it on a regular basis due to the fact that a friend of mine cant eat after concerta
but if you still can/will thats more ideal, It may also worsen the crash)
-Get a good long nights sleep before hand
(Probably the most important thing - improves the effects and greatly reduces the crash)
-You can get an initial kick while still keeping the time-release active by dissolving the coating under the tongue and then swallowing the capsule (20% of the methylphendiate is in the coating - just swallowing the capsule whole will also give you the initial kick but you'll achieve a more fast-acting and intense kick by letting the coating absorb under your tongue)
-Eat foods high in alkaline such as bananas, green vegetables and citrus fruits.
(Apparently helps.. but I personally didnt notice any major improvements but it may work well for you)
You said you've been on Ritalin
before so be aware that the instant release method I described will pretty much turn the concerta into the equivalent amount in Ritalin.
opinion 12 hours is an unnecessarily long time - so maybe you could take a small amount of your dose using in the instant release method over 6 or 8 hours - e.g you could take your 18mg in one go and wait a couple of hours and then take half of your 36 and then a couple of hours later take the final half - just a suggestion, you know how your body better than anyone else does.