Finding handy ways to boost skill checks.

[Lastwell]

For general skills(not specific one).
"Handy" means:
1. Long duration(at least min/level): Skill checks may take a long time.
2. Low spell level: Moment of Prescience is most powerful skill boosting, but not available in most game.
I found...

1.Bestow Insight(Human Racial): +2~+6 bonus and reroll, min/level
2.Heroism: Most basic one. +2 bonus, 10min/level.
3.Moment of Greatness: With heroism, it gives +2 bonus. min/levl
4.Paragon Surge(Half-Elf Racial): It can give skill focus or other skill boost feat. +2~+6, min/level

Are there any others I am missing?

[Mysterious Stranger]

The appropriate stat boosting spells, Bull Strength, Cats Grace, Etc..

[Cevah]

Stone of Good Luck: +1 luck all skill checks [20,000. OK, not low level.]
Ioun Stone(Magenta Prism)<cracked>: +2 comp selected 1/day. [800]
Aid Another: +2

/cevah

[Rycaut]

masterwork tools (non-magical but work with your GM most skills have something - some are specific - i.e. masterwork lock picks for disable device etc)

as others have noted anything that boosts your appropriate stat

remembering that for a long duration skill if you have a long time and there is no penalty for failure you can take 20 (i.e. not for crafting as failure for a magical item means a cursed item) but for most other instances you can take 20 and in almost all cases if you aren't in immediate danger you can take 10

circlet of persuasion - good boost for social skills (and UMD a bit surprisingly)

there are some other items with large boosts many of which have been subject to some degree of errata (mask of stony demeanor for example)

also for many circumstances a simple guidance is helpful - most DMs will let that apply for many skill checks

there are also some special rules worth exploring for some skills - for example though the usual rule is knowledge checks can only be done untrained to a DC10 if you have access to a library or the equivalent the rules say you can make high DC checks (remember you may also be able to take 20 - i.e. you really study - most DMs will say this takes a lot of time but I would usually let players learn a great deal if they have the access to a library with the relevant resources in it and they take them time - as a DM I would generally want them to then get the lore for their troubles.

More broadly in many cases a DM has discretion to allow some circumstance bonuses - indeed in many published modules and PFS scenarios there are specific bonuses spelled out for the GM so I would also generally encourage players to describe what they are doing and how they are doing it (and in some cases there are actually mechanics in the game - for example claiming a sheer wall is a high DC or impossible - but climbing a wall with the aid of a knotted rope is a much lower DC. (and if you have the time taking off your armor and/or dropping your pack may also help)

[hogarth]

The most handy low-level skill boost I can think of is the level 2 bard spell Gallant Inspiration (+2d4 to a failed skill check retroactively). It's obviously not a long duration spell, but it has the enormous advantages that (a) you never have to cast it beforehand and (b) it's much, much less likely to be wasted (consider this -- casting a spell for a +2 bonus to a skill check in advance is going to be a waste 18 times out of 20).