1) Don't do "ranges" (IE: 7.34-7.45), use a mid-point cut off.
For EXAMPLE: I don't consider if my pH is "towards" the low end or "towards" the high end, rather I pick 7.40 as my cut point. If it's above 7.40 even just a 0.01 above, then it's alkalotic and vice versa. Do this for your PaCO2 and HCO3 also - use mid points.
2) I like to draw a 4x3 box and fill in the values starting with the ones I KNOW FOR SURE
For EXAMPLE: My prototype Resp. Alkalosis pt is that hyperventilating-anxiety-stricken-20-something-female that comes into triage. I can easily remember that she has "blown off too much CO2" thus her PaCO2 is LOW. I know it's a respiratory problem because she's hyperventilating. I know that CO2 is a weak ACID so if she's blown off too much of it her pH is HIGH - given that information I can fill in those boxes.
3) Now I look for OPPOSITES. If the above patient is Resp. Alkalosis, then by default, the Resp. Acidosis pt MUST have a LOW pH. Now the prototype pt I use for this is my CARDIAC ARREST pt. We know that they have STOPPED breathing, so too much CO2 is built up (it's a weak acid remember, so that pH MUST be down). So I can now fill in the boxes for that one.
4) Now I move to the Metabolic syndromes. The rules for acidosis and alkalosis pertaining to the pH being HIGH or LOW still hold true - so plug those in under pH.
5) Now look at the HCO3. I KNOW that HCO3 buffers CO2 so if CO2 is an acid, then HCO3 is a base. If my pt is in an acidotic state, then the HCO3 is NOT ENOUGH to compensate and therefore HCO3 is LOW, and thus vice versa. If my pt is alkalotic then the HCO3 is TOO HIGH.
So basically it's a process of elimination and fill in the blanks. When you're done your little table should look a little something like this....