Run at a  water only "mash" on the system first to help you detect problems before you do a real mash.  Combine this with the cleaning and flushes you'll want to do anyhow.  Do NOT assume that the recirc. flow you get during this false mash will be anywhere near the flow you'll get during a mash!  To ball park that and to check for air leask in the plumbing between the tun and pump (they can cause the resulting brew to taste of cardboard due to HSA) , start with tap water, recirc at a highest flow you can to purge any air in the plumbing.  Then restrict the manifold or false bottom at the bottom of the tun.  Keep increasing the restriction until you hear the pump cavitating (see footnote 1), then back off a bit on the flow 'til the noise just stops and look for air bubbles in the recirculating water. I have a short length of 5/8" ID vinyl tubing for this (it's also servers as a coupling) but you can use some clear vinyl on on the discharge side for this.  Stopping the flow via a valve or crimping the hose downstream of the monitoring point will make any air bubbles more noticably since they won't be whizzing past.  You can  apply water to joints in the suction piping to find any air leaks (the air bubbles will be reduced).  Repeat with 170 degF water if you have any sort of unions (and especially flexible tubing in the suction piping) since joints can loosen when the temp. increases due to changes in material properties and via thermal expansion.  With the higer temp. water, you'll note that less restriction is needed to cause caviation which brings up...

Don't feel bad if the grain bed gets stuck.  You'll know because of the reduced recirc flow and maybe the sound of caviatation if it's really stuck.  Unstick it by stopping the flow (and maybe the heat source too), hope the bed "refloats" or stir the bed.  Start recircing from no flow and gradually increase it.  Ways to prevent this are experience (see next item) and/or a sight glass connected to the suction side of the pump.  The later will help you avoid the bane of some RIMS' (at least IMHO)- stuck mashes.  It'll allow you to actually see the degree of grain bed compaction via the amount of pressure available at the pump.  A length of 1/4" ID or so vinyl tubing works and is cheap and usually easy to install.  One caveat tho'- it'll suck air if the abs. pressure at the point of connection is lower than atomspheric so put a valve in the line between the suction piping and the sight glass.  You might find that you can go a below atomspheric with your pump.  How do you find out how little pressure is "too" little?  I guess you could stick a bed on purpose or ....

Save a spent grain bed for expermentation- play around with the variables (false bottom or manifold design, mash thickness, stirring, recirc. flow, bypass flow, and/or etc.) while you analysis how the system responds.   It won't behave exactly like a real mash, but it's plenty close enough give you a good feel for how the system behaves- and you don't risk screwing up a real mash.   I suspose you could add sugar more closely emulate a real mash, but I think that's a bit over the top...

Measure the temp. of the fluid downstream of the heat source.   Vary the flow and see what the effect on the temp. is.  Opinions differ on how hot is "too hot" and even whether it's worth considering.  FWIW, I kill the heater when this temp. is greater than 2 degF above the setpoint...  YMMV.

For your first mash or two:

• If you've never mashed before, write down the steps you'll be going through before hand and refer to it during the mash.  BTW, if you've never mashed before, try doing a nonRIMS mash- you'll then better appreciate RIMS' advantages!
• Choose a recipe/ brew that's easy to mash (so it'll likely be sucessful!) and that yeilds a brew that you can accurately assess for any mashing related problems.  If you have prior all-grain, nonRIMS mashing experience, I recommend brewing something you've brewed before for the first brew or two.  Perferrably something mild and based on pale ale malt (see footnote 2).   Otherwise, consider brewing a simple "lawnmower" ale so that any mashing defects system will be more readily detectable than something more robust:  #8 pale ale malt at 153 degF until an iodine test is negative and the recirc is clear, raise to mashout, hold 10 mins, and sparge.  Go easy on the hops and use a neutral type ale yeast at the lower end of the recommended ferm. temp range (e.g. Nottingham or Wyeast 1056 at 60-65 degF).
• Calculate how much water and it's temp. will be needed for mashing -in to just hit your rest temp..  Consider the mash thickness you want (shoot on the thin side to start with- 1-1.5 qts/#) and the temp. ratings of the components in your system and keep under this.  What you don't want is to depend on a new RIMS to do alot of temp. boosting fast- e.g. going from 120 to 150 degF when you didn't need on a protein rest!  Get in the ballpark and consider having some additional hot infusion water available just in case.   The way I mash-in the first mash on a new RIMS:  Put the calculated volume of water into your RIMS and recirc/heat 'til it's at the target temp. This will also help purge any air from the system and help ID any leaks or other problems.  Ensure your return manifold is submerged while recircing.  Stop the recirc and heat: first kill the heat, then shut off the flow (this helps trap the water in and keeps air out of the plumbing in case you have to raise the return manifold for mashing-in) then shut off the pump.  Add the grains while stirring- make sure they are all wet.  Allow to settle for 5 minutes or so, start the pump and then slowy increase the flow until you're satisfied with the recicr flow or the bed sticks (see "sight glass" above)..
• Monitor the grain bed temp. periodically.  A themometer with a long probe is great for this.
• Kill the heat if the flow stops if you use an electric heater in a tube for a heat source!
• Keep notes and review them occassionally and after system changes.

2.  For the first few mashes, I recommend pale ale malt the base since mashes with it are pretty hard to screw up.  Also they are easy to mash- only one temp. boost will be needed (to mashout- about 168 degF) and the lenght of time it takes to do the boost isn't that important.  Try to avoid speciality malts that are enzyme deficient (since pale ale malt doesn't have alot to spare for other grains).  Also avoid  or flaked grains (they tend to slow recirc. alot).