Transcardial perfusion fixation is the gold standard for fixation as it preserves ultrastructural detail and is preferable to immersion fixation. If you must perform immersion fixation, immediate immersion in fixative is essential. If the dissection requires extra time, finish trimming or blocking the tissue in a glass dish so the tissue is (always) submerged in fixative; otherwise, the tissue will fall apart during sectioning. Most types of tissues are best fixed with 2-2.5% paraformaldehyde and 2-2.5% glutaraldehyde in a 0.1M sodium cacodylate buffer. Phosphate buffer fixatives tend to precipitate out during washing with the sodium cacodylate buffer but can be used during perfusion without a problem. The percentages of glutaraldehyde and PFA vary depending on the tissue type, area of interest, etc.