Plus Damage and Plus Healing Items

"Increases healing done by spells and effects by up to 42."

What does this mean? Counterintuitively, it doesn't mean casting a healing spell grants a random bonus between 1 and 42 for that particular cast. Each healing spell will get a particular, fixed benefit from this "up to 42." This benefit can be calculated mainly from casting time.

ActualBenefit = AdvertisedBenefit * (CastingTime / 3.5)

Spells with a casting time of 3.5 seconds and above receive the full advertised benefit. Spells with less casting time receive a ratio of advertised benefit. Spells with less casting time than 1.5 seconds are still considered to be 1.5 second spells, e.g both 1.5 second spells and instant cast spells get 0.429 (which is 1.5/3.5) of the advertised benefit. DoT or HoT spells have their benefit divided among ticks.

There are two other factors that can reduce the actual benefit:

• Lower Ranks. The formula above only applies to the highest rank of the spell. Having anything less than the highest rank reduces the actual benefit drastically. Blizzard does this so that players won't be tempted to spam low rank spells whose effects might be doubled or tripled by the +Heal or +Damage benefits. This may not be completely true. Testing has shown that all of the shaman LHW spells get the same advantage from +healing items. It may be that there is just a massive dropoff for the lowest (faster casting) ranks of various spells.
• DoT or HoT effects after the initial spell's cast. Some spells have an initial heal or damage followed by a DoT or HoT. That initial component does not receive full benefit as some of it is moved to the HoT or Dot component.
• Channeled spells are treated like DoTs. Arcane Missiles get 20% of the total bonus to each wave of missiles.

Also note:

• If you ever see an item which advertises "increases [...] spells and effects by up to 1 to 42," then it still follows the above formula for the top limit. However, each cast of the spell will give you a random benefit between 1 and what would normally be the actual benefit based on casting time.
• The CastingTime value used to calculate ActualBenefit for a particular spell does not change if a player has a Talent, buff, or debuff affecting cast time.
• Benefits are added to spells before critical hit modifications are made. So, yes, these will raise your criticals.

See this CM thread. This formula has been confirmed correct for Paladins making use of +Healing gear.