choosing surface mount resistor/capacitors
For assembling by hand I offer the following suggestions:


Capacitors:  For the ubiquitous 0.1uF ceramic decoupling caps go with 0603.  They're nonpolar and unlabelled so the small size is not that much of an issue.  You could buy a whole reel and be set for life.  Every tantalum and electrolytic SMT cap is big enough that handling is not an issue.  If you are following a datasheet recommendation to put down 2 or 3 decades of caps near a pin of a high-density package even 0805 will feel very big.
Resistors:  They are labelled down to 0603 (usually).  If you care about reading the markings it's worth checking a few with your own eyes to see what you can read.  I find 0805 much more readable than 0603.  A nice tradeoff for density vs handling is 4 resistor arrays.  A 1206 array is basically 4 0603 resistors side-by-side.  The package is much bigger and the label is much easier to read but the density is even better than 0603 (because there is no gap) and the soldering is only slightly trickier.  These are great for pullups, series/stub terminations, LED arrays, etc.
Jumping over tracks:  It's easy to home-etch a board with a trace going under (between the pads) of a 1206 or 805.  You can manage nearly 10/10 rules cutting under 0603 but you will probably have to make your own land pattern for 0603 to do it (otherwise the pads will be too close).
LEDs:  I find the ones in the tantalum-capacitor style A, B, C, D cases the easiest to orient and solder.  Ones sized like resistors (0603) can be quite hard to orient without magnification.  I usually use a setup where I can probe the LED to light it up to verify which way it's going in.
Other ICs:  SOT-223 and SO-89 are nice for voltage regulators.  SOT-23 is fairly easy to work with in 3-lead packages and slightly trickier in 5 or 6 lead.  SO-8 is easy to work with.  QFP with pitch 0.8 or 1.0mm is pretty easy but fine-pitch QFP (0.65, 0.5mm)  requires a lot of flux and good technique.  QFN, MLF, BGA and the like are very tricky to work with at home and can be difficult to route on 2-layer boards (especially without plated through-holes).  I avoid PLCC when I can because it's neither small nor that easy to solder (it's very easy to bridge pins under the chip and very hard to desolder them).


Mind your ground relief when connecting to surface mount pads.  If you let your plane simply merge with a pad it can be very hard to solder even with a very good iron.

Remember that putting SMT components in your PCB design is almost free.  There's no hole to drill and it's just a modification of the copper and silk masks.  It won't affect the cost of the PCB at all.  If you think you might need another cap or an indicator LED or a pullup resistor just put them all in the design.  You can always DNI them.