It is indeed difficult to reduce to a few fixed rules all aspects of suffix usage, but the following indications, even though they do not cover all possible cases, can answer the most frequent doubts.
The form «-ito»: In relation to nouns, and leaving aside the affective uses discussed in a previous tip, ‘-ito’ denotes smallness in quantity or size, like the following: ‘barquito’, ‘avioncito’, ‘trencito’ and ‘carrito’ clearly denote they are just toys, not the real objects; ‘gatito’, ‘pollito’, ‘conejito’, etc. denote they are referring to the babes of these animals. And similar conditions are true of ‘arbolito’ (sapling), ‘rayita’ (small line) ‘gotita’ (droplet), ‘cochecito’ (baby carriage), etc.
Regarding adjectives, the form «-ito» may indicate the smallness in size of the object being adjectivized, although it most frequently imparts other nuances; it can be an attenuating factor, or a lesser degree in quality, such as in: ‘El día está oscurito' (a little dark). ‘La mezcla quedó espesita’ (somewhat thick). The reality is that these same sentences can also provide more information on quality, so, if the suffix must be interpreted either positively or negatively, it will depend solely on the previous context on which it supports itself.
The form «-in, -ina» has the same value as «-ito, -a», but is much less used. The same goes for the forms «-ico, -ica» e «-iño, -iña», used only occasionally by people that come from regions that do not use that diminutive as their particular trait.
The form «-illo»: can signify smallness in size, but more frequently has to do with minimization of importance, or a disdainful attitude towards the referred object. Thus: 'No se preocupe, sólo es un catarrillo’. ‘Don’t worry, it is just a(n) (insignificant) cold.’ ‘El perrillo que está ladrando ni se ve’. ‘The (insignificant) dog that is barking is not even a big one.’
It has happened very often that words with a permanent autonomous meaning have been created using this same suffix (their current meaning not linked to the word from which they stemmed). This is how the following were formed: pasillo’ (aisle, from “paso”). ‘bombilla’ or ‘bombillo’ (light bulb; both from “bomba”). 'perilla' (knob, from “pera”). ‘patilla’ (sideburn, from ‘pata’). 'el/un cabecilla’ (leader, from “cabeza”). ‘pastilla’ (pill from ‘pasta’). ‘manzanilla’ (chamomile, from “manzana”). ‘platillos’ (cymbals/menu dishes, from “plato”). ‘zapatilla’ (slipper, from “zapato”). ‘planilla’ (payroll, from “plano”). And many others.
The forms «-ete, -eta» and «-uelo, -uela» have a similar value to that of «-illo, -a». These have been used more often than «-illo» --especially «-ete, -a»-- to form words with autonomous meanings such as the ones described above.