Amigurumi is usually crocheted with just one stitch - single crochet stitch (double crochet in UK and Australia) and increasing and decreasing are used to give shape to your toys. Increasing is nothing more than crocheting two or more stitches in the same stitch. And it doesn't matter which stitch you are using, you just need to make extra stitch(es) in the same stitch to increase the number of stitches in a round or row.
The most common increase in amigurumi - two single crochet stitches crocheted into one stitch:
Crocheting more than one stitch in the same hole will stretch it out a little and there will be a small gap under the increase. When using soft and flexible yarns like wool or acrylic, it's usually not that noticeable. But if you want to use firmer and tougher yarns like linen, hemp or cotton, it may be worth to use this little trick.
Here the first single crochet stitch is crocheted into the front loop only and the second one into both loops:
1. Insert the hook under the front loop only ...
2. ... and complete the first single crochet stitch.
3. Then insert the hook in the same stitch, under both loops ...
4. ... and complete the second single crochet stitch.
Because the yarn I used is a soft wool and alpaca blend, I didn't really have that much of a problem to begin with and it's difficult to see the difference in the photos. But if you are noticing your increases leave large holes in the fabric, it might be worth giving this little trick a try.